Over medium heat, sauté onions, garlic & mushrooms in butter until soft, remove from heat and allow to cool
In your food processor, puree liver until smooth
Combine everything together in a large bowl, using your hands to mix well.
Shape meat mixture into meatballs (any size you prefer, we use this scooper)
Bake meatballs until cooked all the way through or an internal temperature of 160 degrees F (about 30 minutes) Rotating them a few times during baking so they don't develop flat spots.
Serve meatballs as desired (with or without tomato sauce)