In large heavy saucepan; melt butter over medium heat. Cook onions and garlic, stirring occasionally, for about 4 minutes or until softened. Add sausage to pan, breaking up. Cook for 6 to 8 minutes or until meat is no longer pink. Stir in rice until coated. Cook for 1 minute.
Add tomatoes, 2 cups (500 ml) stock, parsley, marjoram and pepper. Cover and bring to boil. Reduce heat to low. Simmer for about 20 minutes or until rice is tender and liquid has been absorbed. With fork, stir in parmesan. Season with salt to taste.
Meanwhile, cut peppers in half lengthwise, remove seeds and membranes. In large pot of boiling water, cook peppers, covered, for 5 minutes. Drain and place, cut side down, on racks. Let drain.
Arrange peppers, cut side up, in single layer in large shallow greased baking dish. Fill with rice mixture. Pour remaining stock around peppers. Cover with foil. Bake in preheated 350 F (180 C) oven for 30 minutes. Sprinkle with mozzarella. Bake, uncovered, for 5 minutes or until cheese has melted. Broil for 2 to 3 minutes or until cheese is golden brown.