Preheat the oven to 190 degrees C. Slice the muffins in half and arrange them, cut-side up, in a single layer on a baking tray. Bake for 8–10 minutes or until lightly browned.
Heat a non-stick frying pan and add the sliced sausages. Cook, stirring, for 6 minutes or until the fat is rendered. Using a slotted spoon, transfer the sausage slices to a plate. Drain off most of the fat from the pan.
Add the onion, red or green capsicum and garlic to the pan and cook for about 5 minutes. Stir in the tomato sauce and rosemary. Return the sausage slices to the pan and simmer, stirring occasionally, for 3–4 minutes.
Spoon a little of the sausage mixture onto each muffin half and top with the mozzarella cheese. Bake for 4–5 minutes. Serve hot.