Preheat oven to 375 degrees F. Roll the meatball mixture into 1-inch round balls and place on a non-stick or parchment-lined baking sheet. Bake 20 to 25 minutes, or until cooked through. Meanwhile, melt the butter for the sauce in a pot over medium heat. Add the onion and cook 4-5 minutes. Mix in the flour until well combined. Slowly whisk in the stock. Add the sage and bring to a simmer. Cook 10 minutes. Drain the meatballs well and add them to the sauce. Simmer a few minutes more, season to taste and serve.