Preheat oven to 375 degrees F (190 degrees C) remove stems from mushrooms. Finely chop enough stems to make 1/2 cup (125 mL). Reserve.
Heat oil in a skillet set over high heat. Split the sausage casings and crumble meat into pan. Add the onion, chopped mushrooms and rosemary. Brown, stirring constantly. Drain off fat.
Stir in the breadcrumbs and cheese. Blend in the ketchup and parsley. Spoon into mushroom caps and arrange in a baking dish. Bake for 12 to 15 minutes or until mushrooms are tender and tops are golden. Makes 16 servings.