Mix the flour with yeast, salt and sugar and knead with the water to form a dough until it loosens from the edge of the bowl. Cover and let rise in a warm place for 30 minutes.
Divide the dough into 8 portions and roll out into flat cakes.
For the topping, crumble the feta. Cut or slice the pumpkin into approx. 2 mm thin slices. Peel the onions and cut into very fine wedges.
Mix the sour cream with the egg yolk and season with salt and pepper. Spread on the dough, leaving a 1 cm wide border free. Spread the pumpkin and onions on top. Sprinkle the feta over the top.
Bake at 220 ° C for 10-15 minutes until golden brown. Sprinkle the baked tarte flambée with the thyme.