Prepare the dough. In a large bowl, mix the flour and salt. Add water and olive oil and knead into a smooth dough. Adjust with a little more flour or water if needed. Cover with a clean kitchen towel and let it rest for 30 minutes.
Preheat the oven to 250°C (480°F) or as hot as your oven allows. Place a pizza stone or baking sheet inside to heat up.
Prepare the topping. Mix crème fraîche and fromage blanc in a small bowl. Add nutmeg, salt, and pepper to taste.
Divide the dough into two portions (for two thin tartes). Roll out each portion into a very thin rectangle or circle on a floured surface. Aim for paper-thin consistency. Place the rolled-out dough on parchment paper for easy transfer.
Spread the cream mixture evenly over the dough, leaving a small border. Scatter the bacon lardons and onion slices over the top.
Slide the flammekueche onto the preheated pizza stone or baking sheet using the parchment paper. Bake for 8–12 minutes, or until the edges are golden and crisp.
Serve immediately, garnished with chives or parsley if you wish.