Squeeze out the meat from the sausages and roll it into eight balls. Fry the meatballs in a little olive oil until golden all over. Add the passata, ketchup, fennel, chilli flakes and oregano and bubble without a lid for 8-10 mins until the sauce thickens and the meatballs are cooked through. Season well.
Heat the grill to high. Split the baked sweet potatoes and place them in a small, foil-lined baking dish. Fill the potatoes with the meatballs and sauce, then top with the cheese. Grill for about 2 mins until the cheese melts. Top with the chilli flakes.