Sweet Potato and Pumpkin Gratin Recipe
Active time: 1 hour
Total time: 2½ to 3 hours
Preheat oven to 350°. Lightly oil a 9" x 13" casserole dish and set aside. In a medium pot, heat oil, add garlic and onion, and cook, stirring often, until just tender, about 5 minutes. Whisk in cream, pumpkin, mustard, herbes de Provence, 1 1/2 tsp. salt and pepper, and cook 5 minutes more. Meanwhile, in a large skillet, cook bacon over medium heat, stirring occasionally, until just crisp, 8 to 10 minutes. Using a slotted spoon, transfer bacon to a paper towel–lined plate, then pour the remaining fat in the skillet into the cream mixture, whisking to combine. Set aside.
In a large bowl, use your hands to toss together potatoes, flour and remaining 1 1/2 tsp. salt until coated. Fold in reserved cream mixture and 1 cup Gruyère. Transfer to prepared dish, arranging potatoes in flat layers and pressing down gently to fit.
Lightly oil a large piece of foil and use it to snugly cover dish. Arrange dish on a foil-lined rimmed baking sheet to catch any drips and bake until just tender when pierced in the center with a paring knife, about 1 hour 15 minutes. Uncover; sprinkle with hazelnuts, reserved bacon and remaining 1/2 cup Gruyère; and bake until cheese is golden and bubbly and liquid has thickened, about 30 minutes. Let rest at least 15 minutes before serving.
Make ahead: Recipe can be made up to 4 days in advance. To reheat: Preheat oven to 325°. Place casserole dish in a larger pan and add boiling water to fill halfway up the outside of the dish. Cover gratin with aluminum foil and bake for 30 to 45 minutes.