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Cheesy Potato Gratin Casserole Recipe 

Cheesy Potato Gratin Casserole Recipe

Golden, bubbly, and impossibly cheesy — this Cheesy Potato Gratin Casserole hits every comfort-food note and still looks fancy enough for company
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Prep Time 30 minutes
Cook Time 1 hour 15 minutes
Total Time 1 hour 45 minutes
Servings: 8
Course: DINNER
Cuisine: American
Calories: 256
Ingredients Equipment Method Notes

Ingredients
  

  • Potatoes & Veg:
  • 2½ pounds Yukon Gold potatoes 6–7 medium, sliced into 1/8-inch rounds
  • ½ medium yellow onion thinly sliced
  • Sauce:
  • 3 tbsp salted butter
  • ¼ cup all-purpose flour
  • 1½ cups whole milk or unsweetened almond milk for lighter or dairy-free-ish
  • 8 oz sharp Cheddar about 2 cups, shredded
  • ¾ tsp kosher salt plus more to taste
  • ½ tsp garlic powder
  • Freshly ground black pepper to taste
  • Toppings:
  • 2 oz shredded Gruyère ½ cup
  • 2 oz grated Parmesan ½ cup
  • Chopped fresh chives or parsley for garnish
  • Bold tip: Slice potatoes uniformly use a mandoline if you have one so they cook evenly.

Equipment

  • 9-inch square baking dish (or similar 8×8–9×13 if you prefer thinner layers)
  • Large pot or saucepan for the sauce
  • Mandoline or sharp chef’s knife for even slices
  • Whisk
  • Measuring cups & spoons
  • Aluminum foil and oven mitts
  • FYI: You don’t need expensive tools. A steady hand and a sharp knife work fine — but the mandoline speeds things up and keeps slices pretty.

Method
 

  1. Preheat & prep
  2. Preheat your oven to 375°F (190°C). Spray a 9-inch square baking dish with cooking spray. Peel (or scrub) and slice the potatoes into 1/8-inch rounds, then slice the onion thinly. Arrange potatoes in the prepared dish in three or four rows, leaving a little space so the slices sit slightly slanted. Tuck onion slices between and on top of the potatoes wherever they fit.
  3. Make the cheese sauce
  4. Melt 3 tbsp butter in a medium pot over medium heat. Whisk in ¼ cup flour and cook while whisking for about 30 seconds so the raw flour taste disappears. Slowly stream in 1½ cups milk, whisking constantly to avoid lumps. Raise the heat to medium-high and bring the mixture to a boil, then reduce to medium-low and simmer until the sauce thickens to a creamy gravy, about 5 minutes. Reduce heat to low and stir in 8 oz sharp Cheddar, ¾ tsp kosher salt, ½ tsp garlic powder, and a few grinds of pepper. Taste and adjust salt and pepper if needed.
  5. Assemble & bake (first stage)
  6. Pour the sauce evenly over the arranged potatoes and onions. Cover the pan tightly with foil and bake 45 minutes. This steam-phase softens the potatoes and lets the flavors meld.
  7. Top & finish baking
  8. Remove the foil and sprinkle Gruyère and Parmesan evenly over the top. Return the dish to the oven and bake, uncovered, until the potatoes pierce easily with a fork and the top turns golden brown, 30–45 minutes. For a super-crispy finish, switch the oven to broil for 1–2 minutes — watch closely so it doesn’t burn.
  9. Rest & serve
  10. Allow the gratin to cool 10 minutes so it sets slightly. Garnish with chopped chives or parsley and serve warm.

Notes

Nutrition snapshot (per serving, approx.)

  • Calories: 256 kcal
  • Carbohydrates: 24 g
  • Protein: 13.2 g
  • Fat: 12.6 g (Saturated Fat: 7.3 g)
  • Fiber: 3.5 g
  • Sugar: 1.6 g
Yes, cheese exists here — but the potato gives you good carbs and the dish still fits into balanced family meals. Want fewer calories? Serve smaller portions and pair with a big green salad.