Cheesy Onion Mashed Potato Casserole (Printable)
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 of onions 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 caramelized onions melt into the potatoes and create a sweetness you cannot replicate any other way.
- It reheats beautifully the next day, maybe even better, which makes it perfect for planned leftovers.
02 -
- Do not rush the onion caramelization or you will get soft but pale onions instead of that deep sweet flavor.
- Assemble the whole casserole up to 24 hours ahead and keep it covered in the fridge until you are ready to bake.
03 -
- Warm the milk before pouring it into the potatoes because cold liquid makes them gummy and dense instead of light.
- Resist the urge to overstir after adding the cheese or the potatoes will become gluey rather than fluffy.