How do you prevent meatballs from breaking up when cooking?

I have a recipe for meatballs that are real tender. I use these meatballs in minestrone. Combine 1 pound ground beef; 1 ten ounce package of chopped frozen spinach, thawed and drained on paper towel; 1 egg; 1/2 cup bread crumbs; salt and pepper; 1/2 cup minced parsley; mix well. Shape into 1 inch balls; bake in pan at 350 for 30 minutes until brown. Drain. Add the meatballs last to spaghetti sauce or soup or whatever. They come out real tender.