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 & Pantry
11 - 1 tsp salt, plus more to taste
12 - 1/2 tsp freshly ground black pepper
# Directions:
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 and bring to a rolling boil. 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 thinly sliced onions and sauté, stirring frequently, until deep golden and caramelized, about 15-20 minutes. Stir in 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, Parmesan, salt, and pepper. Mash until completely smooth and creamy with no lumps remaining.
05 - Gently fold half of the caramelized onion mixture into the mashed potatoes. Taste and adjust salt and pepper as needed.
06 - Transfer the potato mixture to the prepared baking dish, spreading it into an even layer. 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 in spots.
08 - Remove from the oven and let rest for 5 minutes. Garnish with fresh chives if desired, then serve warm.
# Expert Advice:
01 -
- The combination of sweet caramelized onions and sharp melted cheese creates a flavor that tastes like it took far more effort than it actually did.
- It reheats beautifully the next day, which means you should absolutely make the full batch even if you are cooking for fewer than six people.
02 -
- Do not rush the onion caramelization because turning up the heat to save time will give you charred edges and raw centers instead of the sweet softness this dish depends on.
- Warming the milk before adding it to the potatoes was a tip I picked up from a diner cook, and it single handedly changed my mashed potatoes from acceptable to memorable.
03 -
- Return the drained potatoes to the hot pot for a minute before mashing so residual heat evaporates any remaining moisture and you get the fluffiest texture possible.
- Reserve a small handful of those caramelized onions to tuck into random spots throughout the mash so diners get surprising little pockets of sweetness in every scoop.