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Preheat your oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or cooking spray.

Heat olive oil in a large skillet over medium heat. Add the diced yellow onion, sliced leeks, and sliced carrots. Sauté for 5-7 minutes until softened. Add the sliced mushrooms and continue to cook for another 5-8 minutes, until the mushrooms have released their liquid and browned slightly. Remove from heat and set aside.

In a medium saucepan, melt the unsalted butter over medium heat. Add the minced garlic and cook for 1 minute until fragrant. Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux.

Gradually whisk in the heavy cream and whole milk until smooth. Bring the mixture to a gentle simmer, stirring occasionally, until it thickens slightly, about 3-5 minutes. Remove from heat and stir in 1 cup of shredded Gruyère cheese, fresh thyme leaves, grated nutmeg, kosher salt, and black pepper until the cheese is melted and the sauce is smooth. Taste and adjust seasoning as needed.

Arrange a single layer of thinly sliced potatoes in the bottom of the prepared baking dish. Spread about one-third of the sautéed vegetable mixture over the potatoes. Sprinkle with about one-third of the rinsed cannellini beans. Ladle about one-third of the cream sauce evenly over the vegetables and beans.

Repeat the layering process two more times, ending with a final layer of cream sauce. Sprinkle the remaining 1/4 cup of shredded Gruyère cheese over the top.

Cover the baking dish tightly with aluminum foil and bake for 30 minutes. Remove the foil and continue to bake for another 20-25 minutes, or until the potatoes are tender, the sauce is bubbly, and the top is golden brown and caramelized.

Remove the gratin from the oven and let it rest for 10-15 minutes before serving. This allows the gratin to set and makes for easier serving.


Preheat your oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or cooking spray.

Heat olive oil in a large skillet over medium heat. Add the diced yellow onion, sliced leeks, and sliced carrots. Sauté for 5-7 minutes until softened. Add the sliced mushrooms and continue to cook for another 5-8 minutes, until the mushrooms have released their liquid and browned slightly. Remove from heat and set aside.

In a medium saucepan, melt the unsalted butter over medium heat. Add the minced garlic and cook for 1 minute until fragrant. Whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux.

Gradually whisk in the heavy cream and whole milk until smooth. Bring the mixture to a gentle simmer, stirring occasionally, until it thickens slightly, about 3-5 minutes. Remove from heat and stir in 1 cup of shredded Gruyère cheese, fresh thyme leaves, grated nutmeg, kosher salt, and black pepper until the cheese is melted and the sauce is smooth. Taste and adjust seasoning as needed.

Arrange a single layer of thinly sliced potatoes in the bottom of the prepared baking dish. Spread about one-third of the sautéed vegetable mixture over the potatoes. Sprinkle with about one-third of the rinsed cannellini beans. Ladle about one-third of the cream sauce evenly over the vegetables and beans.

Repeat the layering process two more times, ending with a final layer of cream sauce. Sprinkle the remaining 1/4 cup of shredded Gruyère cheese over the top.

Cover the baking dish tightly with aluminum foil and bake for 30 minutes. Remove the foil and continue to bake for another 20-25 minutes, or until the potatoes are tender, the sauce is bubbly, and the top is golden brown and caramelized.

Remove the gratin from the oven and let it rest for 10-15 minutes before serving. This allows the gratin to set and makes for easier serving.
