Cheesy Onion Mashed Potato Casserole (Printable)
Creamy mashed potatoes with caramelized onions and melted cheddar, baked golden and bubbly for a comforting side.
# What You'll Need:
→ Potatoes
01 - 2 pounds russet or Yukon Gold potatoes, peeled and cut into chunks
→ Dairy & Cheese
02 - 4 tablespoons unsalted butter
03 - 1 cup whole milk, warmed
04 - 1 1/2 cups shredded sharp cheddar cheese, divided use
05 - 1/2 cup grated Parmesan cheese
06 - 1/2 cup sour cream
→ Vegetables
07 - 2 large yellow onions, thinly sliced
08 - 2 tablespoons olive oil
09 - 2 cloves garlic, minced
10 - 2 tablespoons chopped fresh chives (optional, for garnish)
→ Spices & Pantry
11 - 1 teaspoon kosher salt, plus more to taste
12 - 1/2 teaspoon freshly ground black pepper
# Directions:
01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Place the peeled and chunked potatoes in a large pot and cover with cold salted water. Bring to a boil over high heat, then reduce to a simmer and cook for 15-18 minutes until fork-tender. Drain thoroughly.
03 - While the potatoes cook, heat olive oil in a large skillet over medium heat. Add the thinly sliced onions and sauté, stirring frequently, until deeply golden and caramelized, about 15-20 minutes. Stir in the minced garlic during the final 2 minutes of cooking.
04 - Return the drained potatoes to the pot. Add the butter, warm milk, sour cream, 1 cup of the cheddar cheese, Parmesan, salt, and pepper. Mash until very smooth and creamy.
05 - Gently fold half of the caramelized onion mixture into the mashed potatoes. Taste and adjust salt and pepper as needed.
06 - Spread the mashed potato mixture evenly into the prepared baking dish. Scatter the remaining caramelized onions over the top, followed by the remaining 1/2 cup of cheddar cheese.
07 - Bake uncovered for 20-25 minutes, or until the cheese is fully melted, bubbling, and lightly golden on top.
08 - Remove from the oven and let rest for 5 minutes. Garnish with fresh chives if desired, then serve warm.
# Expert Advice:
01 -
- The caramelized onions disappear into the potatoes like a sweet, savory secret that nobody can quite identify but everyone loves.
- It reheats beautifully the next day, which means you can absolutely make it ahead and enjoy the leftovers for lunch without any guilt.
02 -
- Do not rush the onion caramelization, because turning up the heat to save time will give you crispy burnt edges instead of that deep, sweet softness that makes this dish special.
- Warming the milk before adding it to the potatoes was a game changer I learned the hard way after years of wondering why my mash sometimes turned gluey.
03 -
- Drain the boiled potatoes really well and even shake them in the colander for a few seconds, because excess water is the enemy of rich, creamy mash.
- Reserve a small handful of the best looking caramelized onions for the very top of the casserole so the presentation looks as good as it tastes.