Cheesy Onion Mashed Potato Casserole (Printable Version)
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
# Method:
01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter.
02 - Place peeled and cubed potatoes in a large pot of salted water. Bring to a boil, then reduce heat and simmer until fork-tender, about 15–18 minutes. Drain thoroughly.
03 - While potatoes cook, heat olive oil in a large skillet over medium heat. Add sliced onions and sauté, stirring occasionally, until golden brown and deeply caramelized, about 15–20 minutes. Add 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 in 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 Tricks:
01 -
- Caramelized onions add a sweet depth that makes this feel fancy with almost no extra effort.
- The combination of cheddar and mozzarella creates a stretchy, golden crust that is impossible to resist.
02 -
- Do not rush the onion caramelizing step because that slow browning is where the deepest flavor builds.
- Assembling the whole casserole up to 24 hours ahead and refrigerating it works perfectly, just add a few extra minutes to the bake time.
03 -
- Warm the milk and let the butter come to room temperature before mashing because cold dairy makes potatoes gummy and dense.
- Slice the onions as evenly as possible so some do not burn while others remain raw, patience here pays off in every single bite.