Cheesy Onion Mashed Potato Casserole (Printer-Friendly)

Creamy mashed potatoes with caramelized onions and melted cheese, baked golden and bubbly.

# 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

# 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 over high heat, then reduce to a simmer. Cook 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 deeply golden brown and caramelized, about 15–20 minutes. Add the minced garlic and cook for 1 additional minute until fragrant.
04 - Return the drained potatoes to the pot. Add the butter, warm milk, sour cream, salt, and pepper. Mash until smooth and creamy, ensuring no large lumps remain.
05 - Stir half of the cheddar and mozzarella cheeses into the mashed potatoes. Fold in the caramelized onion mixture, reserving a few tablespoons of onions for the topping.
06 - Spread the potato mixture evenly into the prepared baking dish. Scatter the remaining cheddar and mozzarella cheeses over the top, along with the reserved caramelized onions.
07 - Bake uncovered for 25 minutes, or until the cheese is fully melted, bubbly, and lightly golden in spots.
08 - Remove from the oven and garnish with fresh chives if desired. Allow to rest for 5 minutes before serving hot.

# Expert Advice:

01 -
  • The caramelized onions disappear into the potatoes and make people guess what the secret ingredient is every single time.
  • It reheats beautifully, which means you get to enjoy it for breakfast the next morning straight from the fridge without any guilt.
02 -
  • If you add cold milk to hot potatoes they can seize up and become gummy, so always warm the milk first even if you are in a hurry.
  • Patience is the entire secret to caramelized onions and turning up the heat will not speed things up, it will only burn them and leave you with bitter crispy bits instead of sweet soft ones.
03 -
  • Salt the potato cooking water until it tastes like the ocean because that is your one chance to season the potatoes from the inside out.
  • Let the casserole rest for five minutes after baking so the cheeses set slightly and you get clean beautiful portions instead of a molten slide.

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