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

# Directions:

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 15–18 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 15–20 minutes. Add the minced garlic and cook for 1 additional minute until fragrant.
04 - Mash the drained potatoes together with the 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, then gently fold in the caramelized onions. Reserve a few tablespoons of onions for the topping if desired.
06 - Spread the potato mixture evenly in the prepared baking dish. Top with the remaining cheddar and mozzarella cheeses and any reserved caramelized onions.
07 - Bake for 25 minutes, or until the cheese is fully melted, bubbly, and lightly golden in spots.
08 - Garnish with fresh chives if desired. Serve hot directly from the oven.

# Expert Tips:

01 -
  • The caramelized onions melt into the potatoes so completely that even picky eaters who swear they hate onions will ask for seconds.
  • It reheats beautifully the next day, making it the rare leftover that tastes just as good as the original.
  • You probably already have every single ingredient sitting in your kitchen right now.
02 -
  • Do not rush the caramelization of the onions because that slow browning is where half the flavor in this entire dish comes from.
  • Assemble the whole casserole up to 24 hours ahead, keep it covered in the fridge, and bake it when you are ready, adding about five extra minutes to the bake time.
03 -
  • Warm the milk in the microwave for thirty seconds before pouring it in because cold liquid into hot potatoes causes the starches to seize up into a gluey mess.
  • The casserole freezes well in individual portions wrapped tightly in foil, making it a perfect grab and reheat meal for busy weeknights.

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