In a large saute pan, heat 2 tablespoons oil over medium heat. Add onions, season with salt and pepper, and cook, stirring occasionally, until translucent. Reduce heat to low and continue cooking, stirring occasionally, until onions are golden, 30 to 40 minutes. Set aside.
Meanwhile, cook bacon in a second large saute pan over medium heat until it begins to crisp but still has some fat. Let cool. Cut into 3-inch strips. Set aside. Mix fromage blanc, mascarpone, and creme fraiche together with the flour. Season with salt and pepper as needed. Set aside.
Preheat oven and pizza stone to 500 degrees F. Using plenty of flour, roll a ball of dough into a 10- to 11-inch circle. Pinch crust slightly around the edge to form a rim. Sprinkle pizza peel with cornmeal and transfer dough to peel. Dimple the dough with your fingertips, then brush 1 teaspoon olive oil over the dough, leaving a ½-inch border.
Spread one-quarter of the cheese mixture over the dough, then one-quarter of the onions. Distribute one-quarter of the bacon pieces on top. Slide the tarte onto the pizza stone and bake 12 to 15 minutes, or until the cheese bubbles and the crust is a deep golden brown. Remove the tarte from the oven with the peel; cut into slices. Repeat process with remaining tartes.
Basic Pastry Dough
Pour the flour into a mound on a countertop. Add salt and sugar; toss well. Work butter into flour with fingertips until it forms pea-sized lumps. Add the water, 2 tablespoons at a time, tossing with your fingers, until dough is crumbly and just starting to come together. You may not need all the water.
Form dough into a mound, then use the heel of your hand to push the dough away from you, flattening out lumps. Repeat. Shape into two equal disks, wrap in plastic, and refrigerate at least 1 hour.
Place one disk of dough on a floured pastry board; roll into a circle ⅛-inch thick. Fold dough gently into quarters and transfer to an ungreased 9-inch tart pan. Unfold and gently push dough into pan. Roll out second disk. Cut small decorative holes in center of dough, then transfer to flour-dusted cookie sheet. Refrigerate until needed.
Total time: 1 hour 25 minutes; active time: 25 minutes