Classic Au Gratin Potatoes
Layers of thinly sliced potatoes, a velvety cream sauce, and bubbling, golden cheese on top. Rich, comforting, and the kind of side dish that gets people coming back for seconds before they've even finished their first helping.
Equipment
- 9x13 inch (23x33 cm) baking dish or 2-quart casserole dish
- Mandoline slicer - Highly recommended for even potato slices
- Large saucepan or heavy-bottomed pot
- Whisk
- Sharp chef's knife
- Cutting board
- Box grater or food processor with a grating attachment
- Wooden spoon or silicone spatula
- Measuring cups and spoons
- Aluminum foil
Ingredients
- 2.5 lbs Russet potatoes - 1.13 kg, peeled and thinly sliced to 1/8 inch (3mm) thick
- 1 medium yellow onion - finely diced
- 4 cloves garlic - minced
- 3 tablespoons unsalted butter - 42g, for the sauce
- 3 tablespoons all-purpose flour - 24g
- 1 cup whole milk - 240ml
- 1 cup heavy cream - 240ml
- 1.5 teaspoons kosher salt - 9g, divided
- 0.5 teaspoon black pepper - 1g, freshly ground
- 0.25 teaspoon dried thyme - 0.5g, or 1/2 teaspoon fresh thyme leaves
- 0.25 teaspoon garlic powder - 0.5g
- 1.5 cups Gruyere cheese - 170g, freshly grated, divided
- 1 cup sharp cheddar cheese - 113g, freshly grated, divided
- 2 tablespoons unsalted butter - 28g, softened, for greasing the dish
- Fresh parsley - chopped, optional for garnish
Instructions
- Preheat the oven to 350°F (175°C). Grease a 9x13 inch baking dish with 2 tablespoons of softened butter.
- Peel and slice the Russet potatoes into 1/8 inch (3mm) thick rounds. Place slices in cold water to remove starch, then drain and pat completely dry. Season with 1 teaspoon of salt.
- Melt 3 tablespoons of butter in a large saucepan. Cook the diced onion for 5-7 minutes until soft. Add garlic and cook for 1 minute until fragrant.
- Add the flour and whisk continuously for 1-2 minutes to make a roux. Slowly pour in the milk and heavy cream while whisking constantly to prevent lumps.
- Add the remaining salt, black pepper, dried thyme, and garlic powder. Simmer gently for 3-4 minutes until the sauce thickens enough to coat the back of a spoon.
- Remove from heat and stir in 1 cup of Gruyere and 3/4 cup of sharp cheddar until melted and smooth.
- Arrange half of the potato slices in the prepared dish. Pour half of the cheese sauce over them. Repeat with remaining potatoes and sauce.
- Sprinkle the reserved 1/2 cup Gruyere and 1/4 cup cheddar over the top. Cover tightly with aluminum foil.
- Bake covered for 60 minutes, until potatoes are tender. Remove the foil, increase temperature to 400°F (200°C), and bake uncovered for 20-25 minutes until golden and bubbling.
- Rest for 10-15 minutes before serving. Garnish with fresh parsley if desired.
Notes
- Russet potatoes: Their high starch content helps thicken the sauce naturally as they bake. Avoid waxy varieties.
- Even slicing: A mandoline ensures every slice is exactly the same thickness for even cooking.
- Grate your own cheese: Pre-shredded bags contain anti-caking agents that prevent smooth melting and can make the sauce grainy.
- Dry the potatoes thoroughly: Wet potatoes release water into the sauce, making the finished dish watery.
- Don't skip the rest: Letting the dish rest for 10-15 minutes allows the sauce to set for clean portions.
- Make-ahead: Assemble up to 24 hours in advance, refrigerate unbaked, and add 10-15 minutes to the covered baking time.
- Storage: Refrigerate covered for up to 4 days. Freeze for up to 2 months; thaw overnight in the refrigerator before reheating.
- Reheating: Cover with foil and reheat at 325°F (163°C) for 20-25 minutes. Remove foil for the last 5 minutes to re-crisp the top.
- Cheese variations: Fontina, Parmesan, smoked Gouda, or Emmental are all excellent substitutes or additions.
- Flavor additions: Add a pinch of cayenne pepper or Dijon mustard for heat and sharpness, or fresh herbs like rosemary for a more aromatic profile.