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Potato gratin, or potatoes au gratin, is made even better with the addition of silky caramelized onions. This comfort food classic is a must-have for any family meal.
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One of my favorite side dishes as a kid was a simple preparation of sliced potatoes sautéed with onions. That flavor combination still hits the spot, so this potato gratin recipe became a fast favorite.
Adapted from a reader recipe I tested back when I was a magazine editor, this gratin is loaded with rich caramelized onions and nutty gruyere cheese. The crisp, crackly top gives way to the most tender potatoes that I can’t get enough of!
Perfect for serving as part of the holiday meal with spatchcock turkey or a classic Sunday supper with roast, potato gratin is a dish every cook should have in their back pocket.
Key ingredients for gratin
This is just an overview. For full measurements and step-by-step instructions, scroll down to the printable recipe card, or use the button on the side.
I use Yukon gold potatoes for this gratin. They have a moist texture that almost melts away into the gratin. Don’t use Russet potatoes unless you can’t help it, as they are a bit too dry and mealy.
When it comes to the cheese, I use melty gruyere, the same cheese I use in my asparagus casserole. I love the flavor it adds to the potatoes, adding notes that are fruity, nutty, and earthy all at once. If you can’t find it, good substitutions are comté or emmental.
Use yellow onions, but not sweet onions, in this recipe. Because we are caramelizing them the natural sugars in the onions will be drawn out, and we don’t need the extra sweetness that would come from a Vidalia.
How to make this recipe
First, we need to cook the onions down until they are lusciously soft. Melt butter in a large, heavy pot over medium-low, and add the sliced onions along with salt and pepper.
Cook and stir every so often for 35-45 minutes. Resist the urge to turn up the heat to speed the process! Caramelized onions take time and it is worth it. (Read my full step by step post on how to caramelize onions.)
Once the onions are cooked, add the onion and thyme and cook for 1 minute, just until fragrant. Then, add the heavy cream and increase heat to medium. Bring to a simmer and cook for 3-4 minutes to reduce the cream slighty.
Remove the mixture from the heat and stir in nutmeg, salt, and pepper.
Preheat the oven to 350°F and butter a 9×13-inch casserole dish.
Lay half of the potato slices in overlapping rows (shingles) in the baking dish. Cover with half of the onion and cream mixture, then repeat with the remaining potatoes and onion and cream mixture.
Cover with foil and bake for 45 minutes.
Remove from the oven and remove foil. Sprinkle the gruyere evenly over the top and return the pan to the oven. Bake for 30-35 minutes, until the potatoes are fully tender, the cheese is golden, and the sauce is bubbly.
Let the gratin rest 10-15 minutes, then garnish with fresh thyme and serve.
Recipe tips
Thinly slice the potatoes evenly for better cooking. You don’t need to use a mandoline for this, just slice to about 1/8 inch with a paring knife *.
Take your time with the onions and veer on the side of lower heat if you are worried about burning them. You can make the onions in advance if you want to save time on the day of baking, stopping after adding the garlic and thyme. Then, gently warm with the cream when you’re ready to assemble the gratin.
The gratin cooks first without the cheese to make sure the potatoes are cooked through, without burning the cheese. If your cheese is browning too quickly during the second bake, loosely cover the pan with the foil again.
Don’t forget to let the gratin rest before serving! Not only will this save your tongues from burning, but it will also give the sauce time to settle and thicken.
These caramelized onion potatoes au gratin are something I crave all winter long. You’re going to love it!
Caramelized Onion Potato Gratin
Ingredients
- 2 tablespoons unsalted butter, plus more for buttering the dish
- 2 large yellow onions, halved and thinly sliced
- 1 teaspoon fine sea salt, divided use
- 1/2 teaspoon freshly ground black pepper, divided use
- 2 cloves garlic, minced
- 1 tablespoon fresh thyme leaves, plus more for garnish
- 1 1/2 cups heavy cream
- 2 pounds Yukon Gold potatoes, peeled and sliced 1/8 inch thick
- 1/4 teaspoon freshly grated nutmeg
- 1 1/2 cups shredded Gruyère cheese, (6 ounces)
Directions
- Melt the butter in a large skillet or dutch oven over medium-low heat. Add the onions, 1/2 teaspoon of the salt, and 1/4 teaspoon of the black pepper. Cook, stirring occasionally, for 35-45 minutes, until the onions are very soft and deeply golden. If the bottom of the pan ever looks like it’s getting too dark, add a splash of water and scrape up the brown bits with a spatula, stirring them back into the onions.2 tablespoons unsalted butter, 2 large yellow onions, ½ teaspoon fine sea salt, ¼ teaspoon Freshly ground black pepper
- Add the garlic and thyme and cook for 1 minute, until fragrant.2 cloves garlic, 1 tablespoon fresh thyme leaves
- Add the heavy cream and bring to a gentle simmer over medium heat. Cook for 3-4 minutes, stirring occasionally, until the cream has reduced slightly. Remove from the heat and stir in the nutmeg and remaining 1/2 teaspoon salt and 1/4 teaspoon black pepper.½ teaspoon fine sea salt, ¼ teaspoon Freshly ground black pepper, 1 1/2 cups heavy cream, 1/4 teaspoon freshly grated nutmeg
- Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish.
- Arrange half of the potato slices in overlapping rows in the prepared baking dish, overlapping each slice by about 1/3 to 1/2 and slightly overlapping each row. Pour half of the onion-and-cream mixture evenly over the potatoes.2 pounds Yukon Gold potatoes
- Arrange the remaining potato slices over the first layer in the same overlapping pattern. Pour the remaining onion and cream mixture evenly over the top, making sure it reaches between the slices.
- Cover the baking dish tightly with aluminum foil and bake for 45 minutes.
- Remove the foil and sprinkle the Gruyère evenly over the top. Continue baking for 30-35 minutes, until the potatoes are completely tender when pierced with a knife and the cheese is golden and bubbling.1 1/2 cups shredded Gruyère cheese
- Let the gratin rest for 10-15 minutes. Garnish with fresh thyme and serve warm.
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Nutrition
Nutrition information is provided as a courtesy and is an estimate based on online calculators. Any nutritional information found on Stetted should be used as a general guideline only.
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About Megan
I learned how to cook by exploring seasonal ingredients, and you can too! Meal time shouldn’t be stressful or complicated, and with fresh ingredients and easy methods, I’m here to help you enjoy the time spent in the kitchen. Read more…
