We thought that flammekueche, a savory Alsatian tart traditionally made with smoky bacon and caramelized onions, couldn’t get any better—until we introduced sweet cubes of butternut squash and a layer of tangy goat cheese. A less eggy take on quiche, this hearty tart is excellent as a luncheon showpiece; just serve with a simple mixed-greens salad. We’ve taken some of the pain out of tart-crust prep by calling for a tad more than you’ll use—and a foolproof trick for transferring the dough to the tart pan.
First, make the pastry: In a food processor fitted with a steel blade, pulse flour and salt until combined. Scatter butter pieces over flour and pulse until texture resembles coarse sand. Add 3 tablespoons ice water and process until dough just starts to form clumps, adding up to 2 additional tablespoons, if needed. (Dough will still be loose and crumbly, but should easily form small clumps when pressed together between your fingers.) Transfer dough to a work surface; gather stray crumbs and gently knead dough until just cohesive enough to flatten into a rough disk. Wrap disk in plastic and refrigerate 30 minutes.
Meanwhile, start on the filling: In a large sauté pan over medium heat, cook bacon until browned and starting to crisp, 10–12 minutes. Stir in sage and continue cooking another 30 seconds. Pour the bacon, sage, and bacon grease into a sieve set over a bowl so that the grease can drip down and collect (you’ll be using it to cook the vegetables). Blot bacon with paper towels and set aside.
In the same pan you used to cook the bacon, add 2 tablespoons reserved bacon fat. Cook onions, 2–3 thyme sprigs, 1 teaspoon salt, and pepper to taste over medium heat until soft and moderately browned, 10–12 minutes, stirring. Deglaze pan with ¼ cup white wine, scraping bottom of pan with a wooden spoon. Discard thyme; transfer onion mixture to a large bowl and set aside.
Preheat oven to 375°. When dough is chilled, place it between two double-wide sheets of plastic wrap (or parchment paper) and roll out to form an 11- to 12-inch round, about ¼ inch thick. Gently remove top layer of plastic wrap, leaving dough on the bottom layer. Invert dough onto a 9-½- to 10-inch fluted tart pan with removable bottom; note that you may instead divide the dough among three 3- to 4-inch mini tart pans. Remove plastic wrap, and gently ease dough into the edges of the pan. Remove overhanging dough by running a rolling pin across the top edge. Use extra scraps to patch any tears or thin areas. (You’ll have plenty of extra dough.)
Place pan(s) on a rimmed baking sheet. Lightly mist a small sheet of aluminum foil, cut to fit the center of (each) tart, on one side with cooking spray; place foil, sprayed side down, on top of tart, add a handful of dried beans or pie weights, and bake on middle rack until dough is no longer wet, 20–25 minutes. Carefully remove foil and weights and continue baking until crust is a deep golden-brown, 10–15 minutes longer. (For this step, color is more important than timing, so make sure crust is fully baked.) Let cool in pan until ready to fill.
Now finish the filling. Add 2 additional tablespoons reserved bacon fat to pan (use olive oil if you don’t have enough) and spread squash in a single layer. Cook squash, 2–3 thyme sprigs, 1 teaspoon salt, and pepper to taste over medium heat until softened and moderately browned, 10–12 minutes, stirring occasionally. Deglaze pan with ¼ cup white wine, scraping bottom of pan vigorously. Discard thyme; transfer squash mixture to bowl with onions, toss to combine, and set aside.
Using a rubber spatula, spread goat cheese evenly across bottom of tart shell(s). Transfer squash/onion mixture into tart shell(s) on top of goat cheese, and spread gently from edge to edge. In a small bowl, whisk cream and egg until thoroughly combined; pour evenly over tart filling.
Bake tart(s) on rimmed baking sheet on center rack until just firm and set, about 45 minutes. Sprinkle top(s) evenly with reserved bacon/sage mixture, press down gently with spatula (to ensure an even surface), and bake an additional 15 minutes. Let cool on wire rack at least 15 minutes. Remove from tart pan(s) to cutting board; slice and serve.