In a large saucepan, saute onions in butter until transluscent, about 5 minutes. Add mushrooms and saute an additional 5 minutes.
In a medium bowl, mix the ingredients for the meatballs until just combined. Create approximately 24 meatballs (1 1/2 tablespoons each). Place into the saucepan, with onions and mushrooms, to brown.
Meanwhile in a small mixing bowl, whisk together the ingredients for the sauce. Pour sauce over the meatballs.
Bring the pan to a boil then reduce heat and simmer uncovered for 10 minutes to reduce.
Place lid on the pan and simmer for an additional 20 minutes before serving.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)