Step 1

To make the dough, in a stand mixer with the dough hook, mix the flour, sugar, salt and yeast on medium until well combined, about 15 seconds. With the mixer on low, drizzle in the oil, followed by the water. Knead on medium, scraping the bowl once or twice, until the mixture forms a soft, sticky dough that pulls away from the sides of the bowl, 4 to 5 minutes. Cover the bowl with plastic wrap and let rise at room temperature until doubled, 1 to 1½ hours.

Step 2

Meanwhile, make the toppings. In a 12-inch nonstick skillet over medium, cook the bacon, stirring occasionally, until browned and crisp, about 6 minutes. Using a slotted spoon, transfer to a paper towel-lined plate; set aside. Pour off and discard all but 1 tablespoon fat from the skillet. Add the onions and cook over medium, stirring, until softened and beginning to brown, about 6 minutes. Transfer to a medium bowl and set aside. In a small bowl, stir together the crème fraîche, nutmeg and ½ teaspoon each salt and pepper; cover and refrigerate until ready to use.

Step 3

About 30 minutes into rising, heat the oven to 450°F with a rack in the upper-middle position. Coat a rimmed baking sheet with oil and line it with kitchen parchment, then coat the parchment with oil.

Step 4

When the dough has doubled, lightly flour the counter and turn the dough out onto it. Using a chef’s knife or bench scraper, divide it in half. With your hands, gently pat and flatten each portion into a 12-by-5-inch oval. Transfer the ovals to the prepared baking sheet, spacing them evenly apart. Using your hands, press and stretch the ovals until they measure about 14-by-5 inches, with at least ½ inch between them. Spread half of the crème fraîche mixture in an even layer over one oval, leaving a ½-inch border around the edge. Scatter half of the onions evenly over the crème fraîche mixture, followed by half of the bacon. Top the second oval in the same way.

Step 5

Bake until the edges are golden brown, 18 to 20 minutes, rotating the baking sheet halfway through. Using a wide metal spatula, transfer the tarts to a wire rack. Cool for 10 to 15 minutes, then sprinkle with parsley. To serve, cut crosswise into slices.