Cheesy Onion Mashed Potato Casserole (Printable)

Creamy mashed potatoes with caramelized onions and melted cheddar, baked golden and bubbly for a comforting side.

# What You Need:

→ Potatoes

01 - 2 lbs russet or Yukon Gold potatoes, peeled and cut into chunks

→ Dairy & Cheese

02 - 4 tbsp unsalted butter
03 - 1 cup whole milk, warmed
04 - 1 1/2 cups shredded sharp cheddar cheese, divided use
05 - 1/2 cup grated Parmesan cheese
06 - 1/2 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 kosher salt, plus more to taste
12 - 1/2 tsp freshly ground black pepper

# How-To Steps:

01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - Place peeled and chunked potatoes in a large pot and cover with cold salted water. Bring to a rolling boil, then reduce heat and simmer for 15-18 minutes until fork-tender. Drain thoroughly.
03 - While the potatoes cook, heat olive oil in a large skillet over medium heat. Add the sliced onions and sauté, stirring frequently, until deeply golden and caramelized, about 15-20 minutes. Stir in the minced garlic during the last 2 minutes of cooking.
04 - Return drained potatoes to the pot. Add butter, warm milk, sour cream, 1 cup of the cheddar cheese, Parmesan, salt, and pepper. Mash until completely smooth and creamy.
05 - Gently fold half of the caramelized onion mixture into the mashed potatoes. Taste and adjust salt and pepper as needed.
06 - Spread the potato mixture evenly into the prepared baking dish. Top with the remaining caramelized onions and scatter the reserved cheddar cheese over the surface.
07 - Bake uncovered for 20-25 minutes until the cheese is fully melted, bubbling, and lightly golden on top.
08 - Remove from the oven and let rest for 5 minutes. Garnish with fresh chives if desired, then serve warm.

# Expert Tips:

01 -
  • The caramelized onions add a sweetness that makes this taste like you spent all day cooking, even though the hands on work is surprisingly quick.
  • It reheats beautifully, which means you can make it ahead and enjoy leftovers that taste just as good the next day.
  • Doubled cheese situation means every single bite has that satisfying stretch and savory depth.
02 -
  • Under draining the potatoes is the fastest way to end up with watery mash, so give them a good shake in the colander and let steam escape for a minute.
  • Patience with the onions matters more than anything else here, because rushed onions taste flat and sharp instead of sweet and jammy.
03 -
  • Run your potatoes through a ricer instead of mashing by hand if you want the silkiest texture imaginable with zero lumps.
  • A light sprinkle of smoked paprika over the cheese before baking adds a subtle smoky depth that people will notice but never quite be able to name.

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