Whirl onion, mushrooms eggs and salt in blender. Mix with ground beef and zucchini. Form into small meatballs. Bake on large shallow baking sheet 30 minutes at 350 degrees. Carefully pour off grease, saving gravy-type liquid that forms. Pour into crockpot or warming dish with cream of mushroom soup. Keep warm. Serve as an appetizer or as you would regular Swedish meatballs.