Cheesy Onion Mashed Potato Casserole (Printable)
Creamy mashed potatoes loaded with caramelized onions and melted cheese, baked to golden perfection.
# What You'll 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 to 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 deeply golden and caramelized, about 15 to 20 minutes. Stir in the minced garlic and cook for 1 additional minute.
04 - Mash the drained potatoes together with the butter, warm milk, sour cream, salt, and black pepper until smooth and creamy.
05 - Fold half of the cheddar and mozzarella cheeses into the mashed potatoes, then gently stir 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. Scatter the remaining cheddar and mozzarella over the top, along with any reserved caramelized onions.
07 - Bake for 25 minutes, or until the cheese is melted, bubbly, and lightly golden in 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 makes people close their eyes when they take a bite.
- It reheats beautifully the next day, which means you get two meals of joy from one effort.
02 -
- Do not rush the caramelized onions because low and slow is the only path to that deep, sweet flavor that makes this dish sing.
- Adding cold milk to hot potatoes shocks them into a gluey mess, so always warm your dairy first.
03 -
- Drain the potatoes thoroughly and return them to the warm pot for a minute to evaporate excess moisture before mashing.
- Reserve a spoonful of the starchy potato water to loosen the mash if it turns out thicker than you like.