Combine the flour, eggs, milk and salt and beat to a smooth batter. Set aside at room temperature for one hour.
Meanwhile, sauté the onion in the butter until tender but not browned. Add the mushrooms and cook gently for about fifteen minutes, until most of the liquid has evaporated. Season with the salt and pepper.
Heat a five-inch crêpe pan and grease lightly with butter. Pour two to three tablespoons of the batter into the pan, tilting the pan so that the batter covers the bottom evenly. Cook until the top side is dry and the underneath is lightly browned. Remove from the pan and set aside. Repeat until all batter is used.
Place one tablespoon of the mushroom stuffing in the middle of each crêpe placed browned side up. Roll up the crêpe by tucking in the edges. Dip into the coating eggs, roll in the bread crumbs, and fry gently in the butter, a few at a time, until browned on all sides. Sprinkle with the parsley.