Preheat oven to 425 degrees F (220 degrees C). Lightly spray a baking sheet with cooking spray.
Mix ground beef, ground pork, bread crumbs, onion soup mix, egg, pepper, Italian seasoning, and garlic together in a bowl. Form a 2 to 3-inch meatball around 1 1/2 teaspoons feta cheese.
Place stuffed meatballs on the prepared baking sheet.
Bake in the preheated oven for 10 minutes. Turn meatballs and continue baking until browned and cooked through, 10 to 20 more minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).
Remove from oven and transfer meatballs to a separate pan and tent with aluminum foil for 3 to 5 minutes.
