In a large bowl, thoroughly mix 1 lb ground beef, 1/4 cup minced onion, 1/4 cup breadcrumbs, 1 egg, 2 Tbsp Parmesan cheese (optional), 1 tsp salt, 1/2 tsp pepper, and 1 tsp Worcestershire sauce until well combined. Form the mixture into 25 evenly sized meatballs, each about 1 inch in diameter.
Lightly grease an 8-inch square glass baking dish with nonstick cooking spray. Carefully arrange the preformed meatballs in the prepared baking dish in an even layer.
In a separate small bowl, whisk together 1 cup barbecue sauce and 1/4 cup water. Pour this barbecue sauce mixture evenly over the meatballs until thoroughly coated.
Bake the barbecue sauce-coated meatballs in the preheated toaster oven at 375°F for 35-37 minutes. Gently turn and stir the meatballs every 10 minutes to ensure even baking until fully cooked through without any pink meat remaining inside.
Transfer the tender barbecue baked meatballs in a toaster oven to a serving platter and spoon any remaining sauce in the baking dish over top. Enjoy immediately as a delicious main course or appetizer!