For the tart dough:
Mix the flour, yeast, honey, and salt in a food processor. With the machine running, add the water through the feed tube, followed by the vegetable oil. Process until the dough forms a ball, about 30 seconds.
Turn the dough out onto the counter and knead by hand for 30 seconds, adding extra flour if the dough is sticky. Wrap the dough in plastic wrap and refrigerate for 2 hours, or up to 2 days.
Adjust an oven rack to the lowest position, set a baking stone on the rack, and heat the oven to 500 degrees; let the baking stone heat for 1 hour.
mix the crème fraîche, 1 teaspoon salt, 1/2 teaspoon pepper, and nutmeg together, cover with plastic wrap, and refrigerate until needed.
Cook the bacon in a 12-inch nonstick skillet over medium heat until browned and most of the fat has rendered, about 10 minutes. Transfer the bacon to a paper towel-lined plate, leaving the fat in the skillet. Add the onions and 1/2 teaspoon salt to the fat left in the skillet, cover, and cook over medium heat, stirring occasionally, until the onions are softened and have released their juices, about 10 minutes. Uncover and continue to cook, stirring often, until the onions begin to brown, about 6 minutes; set aside off the heat.
Remove the dough from the refrigerator. Divide it into two equal pieces, forming each piece into a ball. Place one ball on a lightly floured sheet of parchment paper and cover with one large sheet of plastic wrap (or two small overlapping pieces). Using a rolling pin, roll the dough into a 15 by 9-inch oval. Slide the parchment paper with the dough onto an inverted baking sheet.
Spread half of the crème fraîche mixture over the dough, leaving a 1/2-inch border at the edge. Scatter half of the onion mixture and half of the bacon over the top. Slide the parchment with the tart onto the heated baking stone. Bake until the edges of the tart are golden brown and the parchment releases from the tart bottom, 5 to 7 minutes. While the first tart bakes, roll out and assemble the second tart on a sheet of parchment paper using the remaining dough and topping. Cut the baked tarts into about 6 pieces, and serve immediately.