Soak toast in cold water. Peel onions and cut into thin rings. Clean mushrooms and cut into thin slices. Squeeze toast well and knead with minced meat, egg and mustard to a dough.
Season with salt and pepper. Add mushrooms and onion rings. Form 8 meatballs with your hands. Heat oil in a pan. Fry the meatballs on both sides for about 6 minutes on medium heat.