3. Heat oil in a large deep skillet over medium- high heat. Add tomatillo puree and fry, stirring often, until thickened, 6—7 minutes. Carefully add stock and bring to a boil. Reduce heat to medium-low, add meatballs, and simmer until meatballs are cooked through and sauce has thickened, 20—25 minutes. Serve with warm flour tortillas, if you like.