Cheesy Onion Mashed Potato Casserole (Printable View)
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
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 Steps:
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 thinly 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.
04 - Mash the drained potatoes 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. 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, mozzarella, and reserved caramelized onions.
07 - Bake for 25 minutes, or until the cheese is fully melted, bubbly, and lightly golden on top.
08 - Remove from the oven and garnish with fresh chives if desired. Serve hot.
# Expert Advice:
01 -
- Caramelized onions melt into the potatoes and create a sweetness that catches people off guard in the best way.
- It reheats beautifully the next day, making it ideal for leftovers or meal prep.
02 -
- Do not rush the caramelization step because turning the heat up to speed things along only burns the onions and leaves you with bitter results.
- Drying the boiled potatoes in the hot pot for a minute before mashing evaporates leftover moisture and gives you a fluffier, creamier final texture.
03 -
- Taste the mashed potato mixture before you put it in the baking dish because adjusting salt at that stage is your last real chance to get it right.
- Let the casserole rest for five to ten minutes after baking so it sets up and scoops cleanly instead of spreading into a puddle on the plate.