Mix buckwheat flour and 120 g wheat flour with 1/2 teaspoon salt in a bowl, add egg. Gradually stir in 120 ml milk and 350 ml water with a wooden spoon to form a smooth, thin dough.
Let the dough swell for at least 10 minutes.
Melt 1 tablespoon of butter in a saucepan. Stir in 1 tbsp. flour and sauté briefly. Add stock and 200 ml milk while stirring. Bring to the boil, boil for about 5 minutes. Season to taste with salt, pepper and nutmeg.
Wash the spinach, shake dry. Peel the shallots and cut into slices. Clean, clean and halve the mushrooms. Heat the oil in a pan and fry the mushrooms and shallots for about 4 minutes while turning.
After about 3 minutes add the peas. Season with salt and pepper.
Melt 80 g butter. Stir the dough once again. If the dough is not thin enough, stir in some more water. Heat 1 teaspoon of clarified butter in a pan (approx. 24 cm Ø). Pour some batter into the pan and immediately spread thinly with a crêpe batter spreader.
When the dough has set, brush it with some melted butter, turn it over and bake it from the other side until golden brown. Brush the second side with a little butter as well. Bake to the end and take out.
Bake another 7 galettes and stack them on top of each other.
Heat the finished galettes one after the other in 2 pans. Cover galettes with approx. 1/8 béchamel and prepared vegetables. Fold 4 sides of the galettes over the filling and keep warm. Heat another 6 galettes and cover with them.