Preheat oven to 350 degrees F
Over medium heat, saute onions, garlic & mushrooms in butter until soft. remove from heat and let cool.
Puree liver in a food processor, until smooth
combine liver, sautéed onion mixture, and all remaining ingredients in a large bowl, using your hands to mix well.
Shape meat into meatballs. Any size you prefer.
Bake meatballs until cooked all the way through or an internal temp of 160 degrees F (about 30 minutes). Rotate them a few times during baking so they don't develop flat spots.
Serve meatballs as desired, with or without sauce.