French Onion Potato Gratin Recipe

summary

directions

1. Preheat the oven to 375°F. Lightly butter a Christmas Tree Baker casserole dish and set aside.
2. Warm the olive oil in a large skillet over medium-high heat. Add the sliced onions and toss to coat in the olive oil. Stirring occasionally, saute the onions until they are opaque, about 10 minutes. Season generously with salt and pepper and continue to cook until they are golden brown, about 10 more minutes.
3. Lower the heat to medium-low and continue to cook, stirring often, until onions are deep amber brown, about 25 minutes longer. Stir in the balsamic vinegar and cook for 5 more minutes. Turn off the heat and set aside.
4. Using a mandoline or a sharp knife, cut the potatoes into ⅛-inch thick slices. Place half of the potato slices into the prepared dish, cutting a few slices to fit into the tips (the “branches”) of the pan. Season the potatoes lightly with salt and pepper and then scatter the caramelized onions all over the top. Finish with the remaining potato slices and set aside.
5. Melt the butter in a pan over medium heat. Sprinkle the flour over the melted butter and then cook, whisking constantly, for 1 minute. Add the milk, ¼ cup at a time, until it is all incorporated and the sauce thickens, about 2 minutes. Add the cheese, a handful at a time, stirring constantly, until each handful is fully incorporated, about 4 minutes total. Whisk in the nutmeg along with ¼ teaspoon salt and a big pinch of pepper. When you have a creamy sauce, pour it over the potatoes and cover the dish with aluminum foil.
6. Carefully transfer the pan to the middle rack in the oven and bake for 1 hour. Remove the aluminum foil and continue baking until the potatoes are golden brown and the dish is bubbly, about 30 minutes more. Let the potatoes cool for 10 minutes and garnish with the parsley. Serve at once.

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