Cheesy Onion Mashed Potato Casserole (Print)
Creamy mashed potatoes with caramelized onions and melted cheddar, baked into a golden vegetarian casserole.
# Ingredient List:
→ 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 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 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 deeply golden and caramelized, about 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 onions into the mashed potato mixture. Taste and adjust 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 1/2 cup of 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 Suggestions:
01 -
- The caramelized onions melt into the potatoes so every single bite carries a subtle sweetness that people will try to identify but never quite pinpoint.
- It reheats beautifully the next day, making it the rare leftover that tastes even better than the first night.
02 -
- Undercooked potatoes will never mash smoothly no matter how hard you try, so test the largest chunk before draining.
- Adding cheese to potatoes that are too hot can make it separate into oily strands, so let them cool just slightly before mixing.
03 -
- Use a ricer instead of a masher if you want truly lumpless potatoes with minimal effort.
- Low and slow is the only way to caramelize onions properly, so never rush them on high heat or they will char instead of sweeten.