3-4sweet potatoes (about 3 lbs), peeled and sliced into ¼-inch rounds(cut rounds in half if very large)
1teaspoonkosher salt, divided
½teaspooncoarse black pepper, divided
3stripsthick sliced or center-cut bacon, cut into ½ iinch pieces
1large shallot, thinly sliced
2 ½cups2% or whole milk
1 ½cupsheavy cream
6oz.Gruyère cheese, shredded
Preheat oven to 425ºF. Mix sliced sweet potatoes with oil, maple syrup, [1/2 teaspoon] salt and [1/4 teaspoon] pepper. Arrange on large rimmed baking sheet (sheet pan). Roast for 25 minutes.
Meanwhile, fry bacon in a large skillet over medium-high heat until almost crisp. With a slotted spoon, remove bacon from pan and set aside on a paper towel-lined plate.
Reduce heat to medium, add sliced shallot, [1/2 teaspoon] salt and [1/4 teaspoon] pepper to bacon drippings and cook until tender. Stir in flour, cooking and stirring about 1 minute. Add milk, cream, thyme and nutmeg slowly while whisking, increase heat to medium, and cook until mixture thickens slightly, about 5 minutes. Remove from heat and stir in all but 1/2 cup of the Gruyère cheese.
Arrange half of sweet potatoes in greased 9x13 inch baking dish. Cover with half of the sauce and sprinkle with half the bacon.
Arrange remaining sweet potatoes on top, cover with remaining sauce, and sprinkle with bacon.