"Flammkuchen, or tarte flambée, is a pizza-like flatbread topped with cream, onions, and bacon. It originated in the French region of Alsace but it is also popular all over Germany. Like other onion pies, Flammkuchen is mostly eaten in autumn, served with a glass of new white wine."
Place 3 cups plus 3 tablespoons of flour in a large bowl and make a well in the center. Add yeast, 3 to 4 tablespoons of the lukewarm water, and sugar into the well and stir in a small amount of flour from the sides. Set aside until mixture is frothy, about 10 minutes.
Pour in remaining water and olive oil. Mix with a wooden spoon until everything is well combined. Add 1 teaspoon salt and knead vigorously with your hands until dough holds together as a ball.
Loosely cover dough with a clean dish towel and let rise in a warm, dry place until doubled in volume, about 30 minutes.
Prepare the topping while dough rises. Stir 3/4 cup plus 2 tablespoons of heavy cream and sour cream in a bowl until smooth; season with salt, pepper, and parsley.
Preheat oven to 425 degrees F (220 degrees C).
Divide dough into 4 equal pieces. Roll out 2 pieces as thinly as possible into long rectangles on a large piece of parchment, so both will fit on one baking sheet. Fold the sides of the dough pieces to form a 1/4-inch edge all around. Spread cream topping evenly across the dough and top each dough piece with 1/4 of the bacon and the onion rings. Transfer the 2 flammkuchen with the parchment paper onto a baking sheet.
Bake in the preheated oven until golden brown and crisp, about 20 minutes. Repeat with the other 2 dough pieces.