Cheesy Onion Mashed Potato Casserole (Printable Version)

Creamy mashed potatoes with caramelized onions and melted cheese, baked golden and bubbly for a comforting side.

# What You Need:

→ Potatoes

01 - 2 lbs russet or Yukon Gold potatoes, peeled and cubed

→ Dairy

02 - 1 cup whole milk, warmed
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 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, then fold in the caramelized onions. Reserve a few tablespoons of onions 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 on top.
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 get from any spice or seasoning.
  • It reheats beautifully the next day, maybe even better, which makes it perfect for leftovers or potluck planning.
02 -
  • Adding cold milk to hot potatoes shocks them and can make the texture gummy instead of fluffy, so always warm the milk first.
  • If you drain the potatoes and return them to the hot pot for a minute the extra moisture evaporates and the mash becomes noticeably creamier.
03 -
  • Salt your potato cooking water until it tastes like the sea because the potatoes absorb flavor as they cook and you can never fully season them from the outside later.
  • Let the casserole rest for five minutes after baking so the cheeses set slightly and each scoop holds its shape instead of spilling everywhere.

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