1. Combine meatball ingredients in a large bowl and mix well. Form into balls and lay in the bottom of your Crock-Pot insert. Gently place the mushrooms and sliced onions and zucchini in between and on top of the meatballs.
2. Mix sauce ingredients, except for cornstarch and water, in a separate bowl and pour sauce over the meatballs and the sauce.
3. Cook for two and a half hours on high. Add dissolved cornstarch and stir into the sauce. Lower heat to low and cook for another half hour.