Potato Gratin

Potato gratin is a creamy, golden casserole of thinly sliced potatoes baked in a rich cream sauce until the potatoes are tender and the top is bubbling and browned. The key is slicing the potatoes thin enough that they cook through in the oven without turning to mush, and using enough cream and seasoning to make every layer taste good.

Ingredients for Potato Gratin

Potatoes are the base and need to be sliced thin so they soften evenly during baking. Heavy cream and whole milk together create a rich but not overly thick sauce that cooks into the potatoes as they bake. Gruyère cheese melts smoothly and adds nutty depth, while butter enriches the sauce and helps it coat each layer. Garlic and fresh thyme season the whole dish with subtle flavor. Salt and black pepper bring out the taste of the potatoes and cream.

Why This Potato Gratin Works

Thin slices of potato absorb the cream sauce as they bake and soften completely in about an hour, while the top browns and crisps. The starch from the potatoes thickens the cream slightly, so the sauce clings to each layer instead of pooling at the bottom.

Starting the gratin uncovered lets the top brown and crisp, then covering it for the final 20 minutes keeps the insides creamy and prevents the top from burning. The gratin keeps cooking for a few minutes after it comes out of the oven, so pulling it at the right moment prevents the potatoes from falling apart.

Mistakes To Avoid

Thick potato slices stay raw in the center while the edges burn, or they turn to mush before the top browns. Slice them uniformly thin, about 1/8 inch, so they all cook at the same rate.

Too much cream or milk makes a soup, while too little leaves the gratin dry and the potatoes tough. The ratio here is right: the sauce should come three-quarters of the way up the potatoes when you pour it in.

Skipping the 5-minute rest after baking means the sauce is still too thin to hold the gratin together when you cut it, and it falls apart on the plate. The gratin thickens as it cools slightly and the starch sets.

Covering the gratin from the start traps steam and makes the top pale and soft instead of golden and crisp. Start uncovered to brown the top, then cover only in the final 20 minutes if the top is browning too fast.

As an Amazon Associate, we earn from qualifying purchases.

Ingredients

• 3 pounds russet potatoes
• 1 1/2 cups heavy cream
• 1 cup whole milk
• 4 tablespoons butter
• 4 cloves garlic, minced
• 1 1/2 cups Gruyère cheese, grated
• 2 sprigs fresh thyme
• 1 teaspoon salt
• 1/2 teaspoon black pepper

Step-by-step Instructions

1. Heat your oven to 375°F (190°C). Peel and slice the potatoes 1/8 inch thick on a mandoline.
2. In a small saucepan over medium heat, melt the butter and stir in the minced garlic. Cook for 1 minute until fragrant, then pour in the heavy cream and whole milk.
3. Add the salt, black pepper, and fresh thyme sprigs to the cream mixture. Stir to combine and let it warm through for 2 minutes, then remove from heat.
4. Spread one-third of the potato slices in an even layer in a 9×13 inch baking dish. Sprinkle with one-third of the grated Gruyère.
5. Repeat with another one-third of the potatoes and another one-third of the cheese, then top with the final potatoes and remaining cheese.
6. Pour the warm cream sauce slowly over the potatoes until it comes three-quarters of the way up the stack. Discard the thyme sprigs.
7. Bake uncovered at 375°F (190°C) for 40 to 45 minutes until the top is golden brown and the edges are bubbling.
8. Cover the gratin loosely with aluminum foil and bake for another 20 minutes until a thermometer inserted in the center reads 165°F (74°C).
9. Remove from the oven and let rest for 5 minutes before serving.