Thoroughly combine mince, rice, milk, egg, garlic, basil and salt in a bowl. Form the mixture into about 16 meatballs.
Heat olive oil in a frying pan over medium heat, and brown the meatballs well on all sides, about 15 minutes.
Place the pasta sauce, chopped tomatoes, beef stock, red wine, basil, cayenne pepper, bay leaves and garlic in the slow cooker, and stir to combine. Put the meatballs into the slow cooker, and stir gently to cover with sauce. Lay the peppers and onions over the sauce, without stirring.
Cover and cook on Low for 6 to 8 hours. Just before the final hour, gently stir the sauce, being careful not to break apart the meatballs, and remove the bay leaves.