Mix the water and yeast in a large mixing bowl. Add the flour, 1 tsp salt and 1 TBSP of the oil into the bowl and stir until a dough forms and pulls away from the sides of the bowl.
Transfer the dough to a lightly floured work surface and knead by hand for about 5 minutes. Form the dough into a smooth ball and place in a large bowl with 1/2 tsp of oil. Flip the dough ball around in the bowl to cover in the oil. Loosely cover the bowl and place in a warm place to rise for about 1 hour.
Preheat the oven to 450°F.
Season the pork belly with salt & pepper and cook in a skillet over medium heat about 6 minutes or until your desired crispiness (it will cook a little more, but not much, while in the oven). Set the cooked pork belly on a paper towel lined plate and set aside.
Punch down your dough ball and separate into 2 even size balls. Lay out 2 pieces of parchment paper at least 10x13 inches each and place a dough ball on each one. Roll the dough balls into thin rectangles or ovals on top of the parchment paper roughly 10x13 inches. Transfer the flattened dough with the parchment paper to 2 baking sheets.
Combine the Creme Fraiche and nutmeg in a bowl. Spread the Creme Fraiche evenly on top of the dough all the way to the edges then sprinkle the sliced onion and cooked pork belly on top.
Bake the Flammkuchen in the preheated oven for 12 to 15 minutes or until crust is crispy. If baking both Flammkuchen in the oven at the same time rotate the pans halfway through the baking time.
Transfer the Flammkuchen to a cutting board and garnish with chives and/or freshly cracked pepper. Cut into rectangle slices and serve.