Heat the buttermilk in a saucepan. Put the flour, oil and salt in a bowl. Add the buttermilk and use the dough hook of the hand mixer to knead into a smooth dough in about 3 minutes. Shape the dough into a ball, wrap in foil and let rest in a warm place for 180 minutes.
In the meantime, wash the leek, clean and cut into thin rings, cook in a little salted water for about 2 minutes, drain and drain.
Mix the creme-fraiche with the cheese, lemon juice, salt, pepper and nutmeg.
Halve the dough and roll out on 2 baking sheets lined with baking paper and a little flour to make 2 flat cakes. Preheat the oven to 250 degrees.
Spread the cream evenly over the flatbreads. Cut the salmon into strips with the leek and distribute them on the flatbread. Bake both tarte flambée in the hot oven for about 15 minutes Cut the leek and salmon into pieces and serve sprinkled with chives and a little pepper from the mill.