The onions start out filling the skillet, then slowly soften and shrink until they’re deep gold and sweet. That long, gentle cook is what makes caramelized onion mashed potatoes taste like more than plain mash with onions stirred through.
I keep the heat at medium-low and stir every few minutes, lowering it if the edges start getting dark too fast. Once the onions are soft and golden, a little water loosens the browned bits on the skillet so none of that savory sweetness gets left behind.
Yukon Gold potatoes bring a naturally creamy texture, and warm milk and butter turn them into a smooth mash without burying the onion flavor. The finished bowl is buttery, soft, and full of tender golden onion ribbons.
Kitchen Note
After draining, return the potatoes to the hot pot and let them steam for 1 to 2 minutes. That brief pause lets surface moisture escape, so the warm milk and butter make the mash creamy instead of loose.
Save these caramelized onion mashed potatoes for a holiday table or Sunday supper—the long-cooked onions bring sweetness to every buttery bite.
Why These Onions Need to Look Almost Too Dark
Onions that are merely softened taste mild and a little sharp; the deep-golden ones in this mash bring a rounded sweetness that can stand up to the potatoes and butter. They need time for their moisture to cook away and their natural sugars to deepen. Rushing them over higher heat browns the outside before the slices turn fully tender, leaving you with dark edges and a less mellow onion flavor.
Keep the slices moving every few minutes rather than stirring constantly. You want steady contact with the skillet, but not so much stillness that one patch catches. If the onions color faster than they soften, lower the heat and give them more time. Their finished texture should be very soft, with a deep golden color throughout—not crisp or browned only at the edges.
The skillet’s browned bits are worth keeping, too. Deglazing with water loosens them and brings their concentrated flavor back into the onions. Cook off that water before combining the onions with the potatoes; otherwise, extra liquid can loosen the mash.
What the Onion, Potato, and Dairy Each Bring to the Bowl
- Yellow onions — Their balanced sweetness suits a long skillet cook, and thin, even slices soften at a similar pace. White onions work if that’s what you have, but their flavor stays a little sharper; yellow onions are the better choice for a mellow, sweet result.
- Yukon Gold potatoes — Their creamy, buttery flesh mashes smoothly without losing body. Russets can stand in, but they make a fluffier, starchier mash, so handle them gently and stop as soon as the potatoes turn creamy.
- Unsalted butter — It adds richness while letting the salt level stay controlled. A good everyday butter works well here; unsalted matters more than brand because the potato water is already salted.
- Whole milk — Its fat helps the potatoes turn creamy, and warming it keeps it from cooling the hot potatoes. Half-and-half can replace it in the same amount for a richer mash; lower-fat milk will make the result less full-bodied.
- Olive oil and black pepper — The oil works with the butter to cook the onions without relying on butter alone, while black pepper gives the finished mash a gentle savory bite. Freshly ground pepper has a more noticeable aroma, but ordinary ground pepper is fine.
Building Sweet Onions and Creamy Potatoes Without Rushing Either
Let the onions soften before they deepen in color
Heat the olive oil and 2 tablespoons of butter in a 12-inch skillet over medium-low heat until the butter melts. Add the thinly sliced onions with 1/2 teaspoon salt, then cook for 35 to 45 minutes, stirring every few minutes. They should become deeply golden and very soft; if they brown too quickly, lower the heat rather than turning it up or leaving them undisturbed.
Lift the browned bits back into the onions
Pour 1/3 cup water into the skillet once the onions are soft and golden. Scrape the bottom to release the browned bits, which dissolve into the liquid and coat the onions. Keep cooking until the water evaporates, so the onions are moist and glossy rather than sitting in a puddle.
Simmer the potatoes until a fork slides in easily
Put the peeled, 1 1/2-inch potato chunks in a 6-quart pot with 3 quarts cold water and the remaining 1 tablespoon salt. Bring the pot to a boil over high heat, then reduce the heat and simmer for 15 to 20 minutes. The chunks are ready when a fork slips into the center without resistance; if the outside is breaking apart while the middle is still firm, keep the simmer gentle.
Drain the potatoes and return them to the hot pot. Let them steam for 1 to 2 minutes, then move on while they’re still hot.
Mash gently, then fold in the onions
Warm the milk in a microwave-safe measuring cup for 30 to 45 seconds, until it’s hot but not boiling. Add it to the potatoes with the remaining 6 tablespoons butter and mash just until creamy. Stop once the potatoes look smooth and cohesive; working them beyond that can make the texture gluey. Fold in the caramelized onions and black pepper so the onion ribbons stay visible through the mash.
Small Changes for a Different Bowl of Onion Mash
Keep a few onion ribbons on top
Before folding the onions into the potatoes, set aside a small spoonful. Spoon it over the finished mash for a more onion-forward first bite and a visible golden finish; the rest still gets folded through the bowl.
Make a dairy-free version with olive oil and potato water
Replace the butter with olive oil in the same amount, and use reserved potato-cooking water in place of the whole milk, adding it gradually as you mash. The result is less rich and silky, with a more noticeable olive-oil taste, but the onions still bring sweetness and body.
Storage and Reheating
- Refrigerator: Store leftovers in an airtight container for 3 to 4 days. The mash firms up as it chills.
- Freezer: It freezes well for about 1 month in an airtight freezer container. Thaw it in the refrigerator; the texture may look a little uneven until reheated and stirred.
- Reheating: Warm gently in a saucepan over low heat or in the microwave in short bursts, stirring between bursts. A splash of milk loosens the mash if it has thickened; avoid high heat, which can make the texture stiff or gummy. Heat leftovers to 165°F.
Questions That Come Up When Making Onion Mashed Potatoes
Caramelized Onion Mashed Potatoes
Ingredients
Equipment
Method
- Heat the olive oil and 2 tablespoons of the butter in a 12-inch skillet over medium-low heat until the butter melts.
- Add the onions and 1/2 teaspoon of the salt and cook for 35 to 45 minutes, stirring every few minutes, until deeply golden and very soft; lower the heat if they brown too quickly.
- Add 1/3 cup of the water and scrape up the browned bits.
- Cook until the water evaporates.
- Put the potatoes in a 6-quart pot with 3 quarts cold water and the remaining 1 tablespoon salt.
- Bring to a boil over high heat, then reduce the heat and simmer for 15 to 20 minutes, until the potatoes are fork-tender.
- Drain the potatoes and return them to the hot pot; let them steam for 1 to 2 minutes.
- Warm the milk in a microwave-safe measuring cup for 30 to 45 seconds, until hot but not boiling.
- Add the warm milk and remaining 6 tablespoons butter to the potatoes and mash just until creamy.
- Fold in the caramelized onions and black pepper.
