Soak the rolls in the milk and squeeze them out well. Cut the bacon into cubes, leave out. Peel and dice the onion and sauté in the bacon. Add the spinach and egg yolk to the bacon and roll mixture, stir well, season.
Beat egg whites until stiff, fold in. Pour the mixture into a baking dish greased with butter. Sprinkle with the cheese. Put the butter in flakes on top. Put the form on the grid in the oven and bake at 180 ° C for 25-30 minutes.