Place bacon in a skillet and cook over medium-high heat until cooked but not crisp and fat has rendered out, about 8 minutes. Drain in a strainer; reserve the fat.

Place skillet back over medium heat. Add sliced onions. Cook briefly just until they lose their raw edge and soften up slightly, 3 to 5 minutes. Add a teaspoon or 2 of bacon fat if pan seems too dry. Remove skillet from heat and allow to cool to room temperature.

Mix fromage blanc, creme fraiche, nutmeg, salt, black pepper, and cayenne pepper together in a mixing bowl.

Place 1 ball of pizza dough on a well-floured surface. Flatten and roll out to a 10- or 12-inch thin circle. Transfer to a cold cast iron pan. Heat over medium-high heat to pre-cook the bottom of the crust. As dough heats and bubbles appear, deflate them with the tines of a fork so crust ends up thin and crisp (not chewy). When bottom is nicely browned and just about to start getting charred, 5 to 7 minutes, remove from heat. Repeat with remaining dough.

Spread a generous amount of cheese mixture over the crust. Top evenly with some onions and then the bacon. Broil 5 or 6 minutes until edges are browned and starting to lift. Repeat for remaining tartes.