The traditional fillers of the Maultaschen consist of a mix of three essential ingredients:
1) Meat (especially chicken, beef, pork, or bratwurst)
2) Vegetable (i.e. onion, spinach)
3) Sodden pieces of stale bread
These ingredients are commonly a mixture of leftovers and/or fresh ingredients.
The following filling recipe is taken from Mimi Sheraton's The German Cookbook:
Mix spinach and chopped cooked meat together with grated onion. Soak bread slices in milk or water, squeeze dry after 3 to 4 minutes and crumble bread into meat-spinach mixture. Add salt, pepper, nutmeg to taste and mix in eggs. Knead or mix stuffing until thoroughly blended. Put a heaping teaspoonful of filling in center of each square or circle, moisten edges with egg yolk that has been beaten with water and fold in half to form pockets. Press edges together firmly and crimp with your fingers or with a fork. Let stand 2 hours before cooking. Boil 10 minutes in salted water as you would noodles. Drain and serve in chicken or beef broth, or with melted butter and toasted breadcrumbs as a side dish or entree. If you prefer, you can make Maultaschen a little smaller for soup.