Cheesy Onion Mashed Potato Casserole (Print)
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 potatoes in a large pot of salted water. Bring to a boil, then simmer until fork-tender, about 15–18 minutes. Drain well.
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 caramelized, about 15–20 minutes. Add garlic and cook 1 minute more.
04 - Mash the drained potatoes with butter, warm milk, sour cream, salt, and pepper until smooth and creamy.
05 - Stir in half of the cheddar and mozzarella cheeses, then fold in the caramelized onions (reserve a few tablespoons for topping if desired).
06 - Spread the mashed potato mixture evenly in the prepared baking dish. Top with the remaining cheeses and reserved onions.
07 - Bake for 25 minutes, or until cheese is melted and bubbly with lightly golden spots.
08 - Garnish with fresh chives, if desired. Serve hot.
# Expert Tips:
01 -
- The combination of sharp cheddar and caramelized onions makes this taste like it took far more effort than it actually does.
- It reheats beautifully, so making a double batch is never a mistake.
02 -
- Do not rush the onion caramelization step because that deep sweetness is the backbone of the entire flavor profile and shortcuts will taste flat.
- Assembling the whole casserole up to 24 hours ahead and refrigerating it unbaked works beautifully, just add about 10 extra minutes to the bake time.
03 -
- Dry your drained potatoes back in the warm pot for a minute over low heat to evaporate excess moisture before mashing for the fluffiest result.
- Shred your cheese fresh from the block instead of using pre shredded bags because the anti caking additives prevent it from melting into a smooth, gooey layer.