Mix flour (140 g), oil (1 tbsp), salt (1/4 tsp) and water (60 ml) into a dry dough, adding more water (60 ml) if needed. Knead 1 min, rest.
Preheat oven to 240C; line a baking sheet with parchment.
Cut bacon (65 g) into lardons and fry over medium heat until just browning; set aside. Add onions (1/2) and butter (1/2 tbsp) to the pan, cook until softened and just browning.
Mix nutmeg (1), salt (1) and pepper (1) into the creme fraiche (3 tbsp).
Roll dough as thin as possible (~3mm) into a circle/oval on floured surface; transfer to sheet. Spread with the creme fraiche mix (3 tbsp), leaving a bare edge. Top with onions (1/2), bacon (65 g) and gruyere. Bake 6-10 min until edges are crisp and golden.