Add the egg, and then the milk a little at a time, stirring to make a smooth batter. Allow to sit for 30 minutes.
Lightly oil a hot frying pan and spread a thin layer of batter in the pan. Cook until golden brown and turn over and brown the other side. Keep the cooked pancakes in a warm oven while you cook the rest.
Saute the leeks and mushrooms in the butter until the vegetables are softened, and season with the pepper to taste.
Put a portion of the vegetables in the center of each pancake and roll up.