Cook Italian sausage in a skillet over medium heat. Cook until browned. Drain any grease.
In the same pan add olive oil. Add in onion, garlic and red pepper. Cook just until garlic is fragrant 30 seconds -1 minute. Return sausage to the pan. Pour in diced tomatoes and tomato sauce. Add in oregano, fennel and sugar. Mix well. Bring to a bowl and reduce heat to medium low. Cook for 20 minutes or until sauce is reduced and thickened. Season to taste with salt and pepper.
To make a sandwich, spoon mixture on top of bottom bun. Place provolone on top. Place under the broiler and broil just until cheese melts. Repeat with remaining ingredients until four sandwiches are made. Serve.