Cheesy Onion Mashed Potato Casserole (Print View)
Creamy mashed potatoes with caramelized onions and melted cheddar, 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, warmed
03 - 4 tbsp unsalted butter, plus extra for greasing the dish
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 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. 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 black 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 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 in spots.
08 - Garnish with fresh chives if desired. Serve hot.
# Expert Hints:
01 -
- The caramelized onions disappear into the potatoes and create a sweetness that catches people off guard in the best way.
- It reheats beautifully the next day, maybe even better, which is rare for a potato casserole.
02 -
- Under salted potatoes are nearly impossible to fix after the fact, so taste the mash before spreading it into the dish.
- Patience with the onions is everything, rushing them on high heat gives you charred pieces instead of true caramelization.
03 -
- Dry the drained potatoes in the warm pot for a minute before mashing to evaporate excess moisture and concentrate their flavor.
- The dish continues to set as it cools slightly, so that five minute rest is not optional if you want clean servings.