Cheesy Onion Mashed Potato Casserole (Printable)
Creamy mashed potatoes with caramelized onions and melted cheese baked until 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:
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, reserving 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 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 just as good for a quiet lunch as it was for a crowded dinner table.
02 -
- Walking away from the onions while they caramelize almost guarantees they will burn at the worst possible moment, so stay nearby and adjust the heat down if they start browning too quickly.
- Squeezing the drained potatoes back over the warm burner for thirty seconds evaporates excess moisture and makes the final mash noticeably creamier.
03 -
- Taste the mashed potato mixture before you spread it into the dish because once that cheese goes on top, you will not have another chance to adjust the salt and pepper.
- Letting the casserole rest for five minutes after baking makes it easier to scoop and serve without everything sliding apart into a beautiful but messy puddle.