Cheesy Onion Potato Casserole (Print Version)

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

# 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
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 & Pantry

11 - 1 tsp salt, plus more to taste
12 - 1/2 tsp freshly ground black pepper

# How To Make:

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. Cover with cold salted water, bring to a rolling boil, then reduce heat and simmer for 15 to 18 minutes until fork-tender. 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 frequently, until deep golden and caramelized, approximately 15 to 20 minutes. Stir in the minced garlic during the final 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 seasoning with additional salt and pepper as needed.
06 - Spread the potato mixture evenly into the prepared baking dish. Scatter the remaining caramelized onions over the top, then sprinkle with the remaining cheddar cheese.
07 - Bake uncovered for 20 to 25 minutes, or 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 combination of caramelized onions folded into creamy potatoes creates a sweetness that surprises people who think they dont like onion dishes.
  • It reheats beautifully the next day, which means you can absolutely justify eating it straight from the fridge for breakfast.
  • Sharp cheddar and parmesan together give you that deep, savory depth without needing any fancy ingredients.
02 -
  • Do not rush the onion caramelization because pulling them early leaves you with soft but bland slivers instead of the sweet, jammy depth that defines this dish.
  • Warming the milk before adding it prevents the butter from seizing and keeps the mash velvety rather than grainy.
03 -
  • Drain the potatoes thoroughly and even return them to the hot pot for thirty seconds so residual moisture evaporates, because wet potatoes make watery casserole.
  • Taste the mash before you put it in the dish because once that cheese goes on top, adjusting salt becomes impossible.

Go Back