Combine all dough ingredients in a large bowl to form a rough dough.
Knead the dough for 10 minutes until you have a smooth, elastic, stretchy and velvety dough.
Place the dough back into your bowl and cover with a lid.
Leave to rest for 1 to 2 hours at room temperature (or overnight in the fridge).
Preheat the oven and a baking tray to 250°C (the highest temperature possible) 30 minutes before the bake. If you have a pizza stone, preheat the oven and the pizza stone 1 hour before.
Divide the dough into 4 parts (8 parts for smaller sized Flammkuchen). I use a dough scraper to do this.
Shape each part into a ball and leave to rest for 10 minutes.
Combine the crème fraiche and yoghurt in a small bowl, add the nutmeg, salt and pepper and mix well.
Roll out the dough pieces (2-3 mm) and transfer to sheets of baking paper.
Cover with a clean kitchen towel and leave to rest for about 15 minutes.
Fry the bacon strips briefly until almost cooked, don't let them get crispy.
Fry the onion rings in the same pan until slightly browned.
If you are making all 4 (or 8 small) Flammkuchen but baking only one, two (or a few) at a time, don't add the topping to all of them at once. One by one works better as the topping doesn't melt into the dough that way.
Evenly and generously spread the cream mixture onto the dough (you want a really thick coating in order for the finished product not to be too dry), leave a small border around the edge (this will turn golden-brown and crispy).
Scatter the onion rings and bacon on top and sprinkle with thyme.
Slide the sheet of baking paper with the prepared Flammkuchen onto the preheated baking tray and bake for about 12 minutes or until the edges are nicely browned and the bottom is crisp. Repeat until all Flammkuchen are baked.