Cheesy Onion Mashed Potato Casserole (Printer-friendly)
Creamy mashed potatoes with caramelized onions and melted cheese, baked golden and bubbly.
# What You Need:
→ Potatoes
01 - 2 lbs russet or Yukon Gold potatoes, peeled and cubed
→ Dairy
02 - 1 cup whole milk
03 - 4 tbsp unsalted butter, plus extra for greasing
04 - 1 ½ cups shredded sharp cheddar cheese
05 - ½ cup shredded mozzarella cheese
06 - ½ cup sour cream
→ Vegetables
07 - 2 large yellow onions, thinly sliced
08 - 2 tbsp olive oil
09 - 2 cloves garlic, minced
10 - 2 tbsp chopped fresh chives (optional, for garnish)
→ Spices & Seasonings
11 - 1 tsp salt, plus more to taste
12 - ½ tsp freshly ground black pepper
# How to Make It:
01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter.
02 - Place the peeled and cubed potatoes in a large pot of salted water. Bring to a boil, then reduce heat and simmer until fork-tender, about 15 to 18 minutes. Drain thoroughly.
03 - While the potatoes cook, heat olive oil in a large skillet over medium heat. Add the sliced onions and sauté, stirring occasionally, until golden brown and deeply caramelized, about 15 to 20 minutes. Add the minced garlic and cook for 1 additional minute.
04 - Mash the drained potatoes with butter, warm milk, sour cream, salt, and pepper until smooth and creamy.
05 - Stir half of the cheddar and mozzarella cheeses into the mashed potatoes. Fold in the caramelized onions, reserving a few tablespoons for the topping.
06 - Spread the potato mixture evenly into the prepared baking dish. Top with the remaining cheddar and mozzarella cheeses and the reserved caramelized onions.
07 - Bake for 25 minutes, or until the cheese is melted, bubbly, and lightly golden in spots.
08 - Garnish with fresh chives if desired. Serve hot.
# Expert Tips:
01 -
- The caramelized onions melt into the potatoes so deeply that every bite carries a sweet, savory richness that regular mashed potatoes never achieve.
- It reheats beautifully the next day, making it the rare casserole that tastes even better for lunch.
02 -
- Patience with the onions is non negotiable: rushing them on high heat creates charred bitterness instead of sweet caramelization.
- Assembling the whole casserole up to 24 hours ahead and refrigerating it unbaked works perfectly, just add five extra minutes in the oven.
03 -
- Warm your milk and let your butter come to room temperature before mashing so cold dairy does not shock the hot potatoes into a stiff, gluey mess.
- Reserve a quarter cup of the caramelized onions and tuck them into concentrated pockets throughout the mash instead of mixing everything uniformly for bursts of intense sweetness in certain bites.