Cheesy Onion Mashed Potato Casserole (Printable Version)

Creamy mashed potatoes with caramelized onions and melted cheese, baked until 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, 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, for garnish (optional)

→ 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 over high heat, then reduce to a simmer and cook until fork-tender, 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 brown and caramelized, 15 to 20 minutes. Add the minced garlic and cook for 1 additional minute until fragrant. Remove from heat and set aside, reserving a few tablespoons of onions for the topping.
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 - Fold half of the cheddar and mozzarella cheeses into the mashed potatoes, followed by the caramelized onion mixture. Stir gently until evenly distributed throughout.
06 - Transfer the potato mixture to the prepared baking dish, spreading it into an even layer. 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 Hints:

01 -
  • The caramelized onions disappear into the potatoes and make people close their eyes when they take the first bite.
  • It reheats beautifully the next day, which means you will secretly hope there are leftovers.
02 -
  • Do not rush the onions because under-caramelized onions taste like a mistake, not a feature, and you cannot fake that sweetness once they are buried in potatoes.
  • Assemble the whole thing up to 24 hours ahead and keep it covered in the fridge, then just add five extra minutes to the bake time straight from cold.
03 -
  • Warm the milk and let the butter come to room temperature before mashing because cold dairy shocks the hot potatoes and makes them gluey.
  • Salt the potato cooking water until it tastes like the ocean because that is your only chance to season the potatoes from the inside out.

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