French Onion Potato Gratin: Rich, Creamy One-Dish Bake

This French Onion Potato Gratin turns everyday ingredients into a rich, comforting dish. Thinly sliced potatoes are layered in a creamy French onion sauce, topped with Swiss cheese, and baked until golden and bubbling.

The result is a side dish that feels special enough for a holiday table yet simple enough for a weeknight. The sauce seeps into every potato slice, creating a silky texture with a savory onion flavor.

French onion soup meets classic potato gratin in this easy, crowd-pleasing bake. Instead of a traditional béchamel, this gratin uses a rich French onion sauce—caramelized onions simmered in broth and cream—to coat thin potato slices. Topped with nutty Swiss cheese, the dish bakes until the top is golden and the potatoes are fork-tender.

The result is a creamy, savory side that pairs beautifully with roasted meats, poultry, or a simple green salad. It’s a make-ahead friendly recipe that can be assembled early and baked when ready, making it a reliable choice for entertaining.

Ingredients

Why this recipe works

The French onion sauce does double duty: it adds deep umami flavor from caramelized onions and provides the moisture needed to cook the potatoes. As the gratin bakes, the sauce reduces and concentrates, coating each potato slice in a silky, savory glaze. This method ensures every bite is packed with onion flavor, without the need for a separate béchamel or cheese sauce.

Thinly sliced potatoes cook evenly and absorb the sauce beautifully, creating a tender, creamy texture. Overlapping the slices in rows ensures they cook uniformly and present nicely when served. The Swiss cheese topping melts into a golden, slightly crisp crust that contrasts with the soft potatoes underneath, adding both texture and a nutty flavor that complements the onions.

Notes and substitutions

Choose waxy potatoes like Yukon Gold or red potatoes—they hold their shape during baking and give a creamy texture. Avoid starchy russets, which can break down and become mushy.

For the sauce, use a high-quality store-bought French onion sauce or make your own by simmering caramelized onions with beef broth and a splash of cream until thickened. The sauce should be pourable but not watery.

Swiss cheese is traditional, but Gruyère, Emmental, or fontina are excellent substitutes. For a sharper flavor, use extra-sharp cheddar or a mix of cheeses. A mandoline makes slicing the potatoes quick and uniform, but a sharp knife works well too.

Use a baking dish that holds the potatoes in a single layer—about 9x13 inches is ideal. The gratin can be assembled up to a day ahead; cover and refrigerate, then bake as directed, adding 10 minutes to the covered baking time.

• gold potatoes: Yukon Gold or other waxy potatoes work best; they hold their shape and give a creamy texture.
• French onion sauce: Use a store-bought French onion sauce or make a quick version by simmering caramelized onions with beef broth and a splash of cream.
• Swiss cheese: Gruyère, Emmental, or fontina are excellent substitutes for a nutty, melty topping.

Equipment

• Large rectangular baking dish
• Sharp knife or mandoline
• Cutting board
• Measuring cups
• Small pitcher or bowl
• Aluminum foil
• Serving spoon