Cheesy Onion Mashed Potato Casserole (Printable Version)
Creamy mashed potatoes with caramelized onions and melted cheddar, baked golden and bubbling.
# 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 & Pantry
11 - 1 tsp salt, plus more to taste
12 - 1/2 tsp freshly ground black pepper
# Step-by-Step Instructions:
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 golden and deeply caramelized, about 15-20 minutes. Stir in the minced garlic during the final 2 minutes of cooking and remove from heat.
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 - Fold half of the caramelized onion mixture into the mashed potatoes. Taste and adjust salt and pepper as needed.
06 - Spread the mashed 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 20-25 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.
# Tips from hearthlykitchen:
01 -
- The caramelized onions sink into the potatoes and create this sweet, savory depth that tastes like you spent all day cooking.
- It reheats beautifully, so making a double batch is never a mistake worth regretting.
02 -
- Undercooked onions will taste sharp and raw against the soft potatoes, so do not rush that caramelization step.
- Warming the milk before adding it keeps the mash light and fluffy instead of dense and gluey.
03 -
- Shred your own cheese from a block because pre shredded has coatings that prevent it from melting smoothly.
- Let the potatoes sit in the colander for a minute or two and shake it gently to remove every last drop of water.