Potatoes au gratin is layers of thinly sliced spuds together with a creamy sauce and melted cheese. A rich bechamel sauce that is thickened with a roux is spread between the potatoes which creates a luxurious texture. This casserole recipe is baked until hot, bubbly, and the cheese is golden brown. It’s the potato version of macaroni and cheese that disappears fast the moment it’s served!
Cheesy potatoes au gratin is a classic side dish that’s easy to prepare and is always a crowd pleaser. Except if the cream curdles or the potatoes aren’t fork-tender after baking, but with the right guidance, I’m going to show you how these common problems can be easily avoided.
This recipe uses multiple layers of very thinly sliced Yukon gold potatoes topped with a roux-based sauce to stabilize the milk in the dish. Aged sharp cheddar and buttery gruyere cheese mix together in the sauce and get sprinkled on the potatoes. It’s a comforting dish that can be prepared ahead of time and enjoyed later.
What’s the Difference Between Scalloped Potatoes and Au Gratin Potatoes?
Scalloped potatoes are traditionally cut into thicker rounds, about ¼-inch, layered and baked in a casserole dish. Cream, milk (or both) are added, along with some aromatics like garlic, onions, and herbs. Au gratin potatoes, on the other hand, are thinner, about ⅛-inch thick pieces and cheese is added to the recipe. You might also sometimes see breadcrumbs sprinkled on top for extra crunch. The culinary terms are often used interchangeably which causes a bit of confusion.
How to Make Au Gratin Sauce
The basic technique used to create a milk sauce is using a thickening agent, like a roux. This recipe uses equal parts by volume of butter and flour which cooks into a paste, then gradually incorporating milk while simmering until the thickness can nicely coat a spoon. The result is a bechamel with cheese whisked in. This is technique is also used in macaroni and cheese, but we’re using potatoes instead of pasta. If made correctly, the sauce should spread easily between the potato layers instead of pooling at the bottom of the baking dish.
How to Prevent the Sauce From Curdling
If you only add milk to the potatoes, the proteins are at risk for curdling over time. Once the protein hits a temperature of 180ºF and above they begin clumping together. That’s why recipes often use milk AND cream because the increased amount of fat helps reduce curdling since there’s less protein. Considering the potatoes are cooked at 400ºF for nearly an hour to tenderize, it’s best to build in a little safety cushion with a roux. The creamy texture added to the sauce is just a tasty bonus!
The Best Potatoes to Use for Au Gratin
Choose a medium-sized waxy type of potato like Yukon gold or gold for au gratin. Once cooked the flesh has a nice buttery texture, but it holds its shape well when scooping out each serving. Russets are my second choice because, with their delicate texture, they tend to absorb the sauce and become too creamy. I prefer to use Russet potatoes for mashed potatoes instead.
The best combination of cheese to use for the au gratin potatoes is gruyere and sharp cheddar. Gruyere is a medium-hard cheese that has a nutty taste and hint of sweetness. It has excellent melting properties, and I always use it when making French onion soup. Sharp cheddar has a stronger aged flavor and a semi-hard texture. Together they melt effortlessly in the bechamel sauce and create a nice gooey, browned crust on top of the potatoes.
Key Tips for Making Potatoes Au Gratin
- Peel and slice the potatoes into ⅛-inch thick pieces for even cooking. I recommend using a mandoline.
- Overlap half the potatoes to create a base, spread the sauce, cheese, and repeat.
- Infuse more flavor by adding gruyere and cheddar cheese into the sauce, and sprinkling extra between the layers.
- Cover the baking dish with foil to help gently cook the potatoes.
- Finish cooking the potatoes uncovered to create a melty, golden cheese crust.
Use a large serving spoon to dig into the layers of the cheesy potatoes. Each bite melts in your mouth, and the flavors of the garlic and nutmeg infuse into each piece. This side dish can be assembled ahead of time and then baked right before serving. My family also enjoyed leftovers of the potatoes and it still tasted delicious warmed up!
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Starches Help to Stabilize the Cream Sauce
The natural starches in flour help to stabilize the milk emulsion in the sauce, preventing the separation of the oils and fat. When the starches are heated, they’re able to hold onto some of the water in the milk, swell, and create a viscous mixture. By incorporating a roux into a dairy-based sauce will thicken and stay creamy once baked in the oven.
Potatoes Au Gratin
- 1/4 cup unsalted butter, (60g, 2 ounces)
- 1 tablespoon minced garlic, (8g)
- 1/2 cup finely chopped yellow onion, (72g)
- 1/4 cup all-purpose flour, (30g, 1 ounce)
- 2 cups whole milk, (480 ml)
- 3/4 teaspoon kosher salt, (4g)
- 1/2 teaspoon black pepper
- 1/8 teaspoon nutmeg
- 2 pounds yukon gold potatoes, (908g) peeled and cut into 1/8-inch thick slices
- 1 cup shredded gruyere cheese, (117g, 4 ounces)
- 1 cup shredded sharp cheddar cheese, (117g, 4 ounces)
- 2 teaspoon chopped chives, (1g)
- Set the oven rack to the middle position and preheat oven to 400ºF.
- Melt butter in a large saute pan over medium-low heat. Add garlic and onions, saute until fragrant, 2 minutes.
- Add flour, whisk to combine, stir and cook for 2 minutes.
- Gradually whisk in milk and cook until thickened over medium heat, 2 to 3 minutes.
- Turn off heat and whisk in salt, pepper, nutmeg, 1/2 cup gruyere, and 1/2 cup sharp cheddar cheese.
- Grease the bottom and sides of an 8 by 8 by 2-inch casserole dish.
- Layer half of the potatoes, in overlapping rows in the dish. Spread half of the cheese sauce over the potatoes. Sprinkle 1/4 cup gruyere and 1/4 cup cheddar over the sauce.
- Layer the remaining potatoes, followed by cheese sauce, and sprinkle with the remaining gruyere and cheddar cheese.
- Cover potatoes au gratin with foil and place on a baking sheet. Bake for 30 minutes, remove the foil and bake until potatoes are tender and cheese is browned and bubbly, 20 to 25 minutes.
- Garnish potatoes with chives.
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