Remove sausage skins. Cut into ½-inch pieces.
Place sausage in a large kettle. Place kettle over low heat.
Add onion. Sauté, without butter or oil, over low heat until golden.
Add stock. Bring to a simmer.
Add rice and tomatoes. Continue simmering until rice is cooked, about 15 - 20 minutes.
Add spinach. Cook for a few more minutes until spinach has wilted.
Taste. Add salt and pepper, to taste.
Serve hot with a sprinkling of mozzarella cheese over top.
