Cheesy Onion Mashed Potato Casserole (Printable version)
Creamy mashed potatoes with caramelized onions and melted cheddar, baked golden and bubbly.
# What You’ll 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 & Seasonings
11 - 1 tsp salt, plus more to taste
12 - 1/2 tsp freshly ground black pepper
# How to Make It:
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 boil over high heat, then reduce to a simmer and cook 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 thinly sliced onions and sauté, stirring frequently, until deep golden and caramelized, about 15-20 minutes. Stir in the minced garlic during the last 2 minutes of cooking. Remove from heat and set aside.
04 - Return drained potatoes to the pot. Add butter, warm milk, sour cream, 1 cup of the cheddar cheese, all of the Parmesan, salt, and pepper. Mash until very 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 evenly with the remaining 1/2 cup cheddar cheese.
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 -
- It turns humble pantry staples into something rich enough to steal the spotlight from any main course.
- The caramelized onion layer adds a sweetness that makes this taste like you spent far more effort than you actually did.
02 -
- Under salted potatoes are the most common mistake with this dish, so be generous during the boiling stage and taste the mash before baking.
- Adding cold milk or sour cream to hot potatoes drops the temperature and can make the texture gluey instead of fluffy.
03 -
- Pat the boiled potatoes dry in the colander for a few seconds before mashing to remove every last drop of water.
- The slower you caramelize the onions the sweeter and deeper the flavor becomes, so never rush that step on high heat.