In large skillet, cook sausage and onion over medium heat until sausage is no longer pink, stirring occasionally. Drain.
In 3 1/2- to 4-quart slow cooker, combine cooked sausage and onion, bell pepper, mushrooms and tomato paste; mix well. Top with pizza sauce.
Cover; cook on low setting for 6 to 8 hours.
About 10 minutes before serving, heat toast as directed on package. Stir sausage mixture. Spoon about 3/4 cup sausage mixture onto each warm toast slice. Sprinkle each with 2 tablespoons cheese.