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In a large, heavy-bottomed pot or Dutch oven, melt 3 tablespoons of unsalted butter over medium-low heat. Add the thinly sliced white onions and 1 teaspoon of salt. Cook, stirring occasionally, for 30-40 minutes, or until the onions are deeply golden brown, very soft, and significantly reduced in volume. This is the caramelization process.

Once the onions are caramelized, add the minced garlic and 1 teaspoon of chopped fresh thyme to the pot. Cook for 1 minute until fragrant.

Add the remaining 3 tablespoons of unsalted butter to the pot with the caramelized onions. Once melted, sprinkle in the all-purpose flour. Cook, stirring constantly, for 2-3 minutes to create a roux, ensuring the flour is fully incorporated and lightly toasted.

Gradually whisk in the whole milk and heavy cream, a little at a time, until smooth and no lumps remain. Bring the mixture to a gentle simmer, stirring constantly, until the sauce thickens to a creamy consistency, about 5-7 minutes. Remove from heat.

Stir in the 1/2 teaspoon of black pepper and the Dijon mustard into the cream sauce. Taste and adjust seasoning with additional salt and pepper if needed.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Begin layering the gratin: Spread a thin layer of the creamy caramelized onion sauce on the bottom of the prepared baking dish. Arrange a single layer of thinly sliced potatoes over the sauce, slightly overlapping. Sprinkle about 1/2 cup of grated hard European cheese over the potatoes.

Repeat the layering process: sauce, potatoes, cheese, until all ingredients are used, ending with a layer of potatoes on top. Ensure the potatoes are evenly distributed.

Sprinkle the remaining 1 teaspoon of chopped fresh thyme over the top layer of potatoes. Cover the baking dish tightly with aluminum foil.

Bake the covered gratin for 60-75 minutes, or until the potatoes are tender when pierced with a fork. The baking time may vary depending on the thickness of your potato slices.

Remove the gratin from the oven and carefully remove the foil. Increase the oven temperature to broil (or grill setting). Sprinkle the remaining 1 cup of grated hard European cheese evenly over the top of the gratin.

Return the gratin to the oven, uncovered, and broil for 3-5 minutes, watching carefully, until the cheese is melted, bubbly, and golden brown with crispy edges. Be careful not to burn it.

Let the gratin rest for 10-15 minutes before serving to allow it to set and make for easier serving. Garnish with additional fresh thyme if desired.


In a large, heavy-bottomed pot or Dutch oven, melt 3 tablespoons of unsalted butter over medium-low heat. Add the thinly sliced white onions and 1 teaspoon of salt. Cook, stirring occasionally, for 30-40 minutes, or until the onions are deeply golden brown, very soft, and significantly reduced in volume. This is the caramelization process.

Once the onions are caramelized, add the minced garlic and 1 teaspoon of chopped fresh thyme to the pot. Cook for 1 minute until fragrant.

Add the remaining 3 tablespoons of unsalted butter to the pot with the caramelized onions. Once melted, sprinkle in the all-purpose flour. Cook, stirring constantly, for 2-3 minutes to create a roux, ensuring the flour is fully incorporated and lightly toasted.

Gradually whisk in the whole milk and heavy cream, a little at a time, until smooth and no lumps remain. Bring the mixture to a gentle simmer, stirring constantly, until the sauce thickens to a creamy consistency, about 5-7 minutes. Remove from heat.

Stir in the 1/2 teaspoon of black pepper and the Dijon mustard into the cream sauce. Taste and adjust seasoning with additional salt and pepper if needed.

Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.

Begin layering the gratin: Spread a thin layer of the creamy caramelized onion sauce on the bottom of the prepared baking dish. Arrange a single layer of thinly sliced potatoes over the sauce, slightly overlapping. Sprinkle about 1/2 cup of grated hard European cheese over the potatoes.

Repeat the layering process: sauce, potatoes, cheese, until all ingredients are used, ending with a layer of potatoes on top. Ensure the potatoes are evenly distributed.

Sprinkle the remaining 1 teaspoon of chopped fresh thyme over the top layer of potatoes. Cover the baking dish tightly with aluminum foil.

Bake the covered gratin for 60-75 minutes, or until the potatoes are tender when pierced with a fork. The baking time may vary depending on the thickness of your potato slices.

Remove the gratin from the oven and carefully remove the foil. Increase the oven temperature to broil (or grill setting). Sprinkle the remaining 1 cup of grated hard European cheese evenly over the top of the gratin.

Return the gratin to the oven, uncovered, and broil for 3-5 minutes, watching carefully, until the cheese is melted, bubbly, and golden brown with crispy edges. Be careful not to burn it.

Let the gratin rest for 10-15 minutes before serving to allow it to set and make for easier serving. Garnish with additional fresh thyme if desired.
