In a bit of water, covered, cook Les Cultures de chez nous sliced leeks and sliced mushrooms for 5 minutes, then drain. Set aside.
Melt butter and add flour. While stirring, slowly add milk, salt, pepper and nutmeg. Cool down.
Add leeks and mushrooms to the béchamel sauce.
Add the béchamel sauce, leek, and mushroom mixture to each crepe and roll. Lay the crepes side by side on a plate. Spread with grated cheese.
Cook for 15 minutes in the oven preheated to 375°F (190°C). If desired, finish baking with broil.