Preheat oven to 400 degrees. prepare a baking sheet by lining with foil and spraying with nonstick cooking spray.
Heat a small amount of oil in a small skillet and cook onions until translucent. Add garlic and cook for about a minute, until fragrant. Set aside until cool enough to handle.
Once onions have cooled slightly, combine with sausage, egg, breadcrumbs, Parmesan cheese in a large bowl. Use your hands for best results and so you don't over-mix the meat, which makes it tough.
Divide the meat mixture into 24 equal sized meatballs and place on the foil lined baking sheet, at least an inch apart.
Bake at 400 degrees for about 18-20, or until no longer pink.
Serve with your favorite pasta and sauce.