In 12-inch skillet, cook ground beef and onions over medium-high heat 5 to 7 minutes, stirring frequently, until beef is thoroughly cooked; drain.
Spray 3 1/2- to 4-quart slow cooker with cooking spray. Spoon beef mixture into slow cooker. Stir in bell pepper, pizza sauce, pepperoni, basil and oregano.
Cover; cook on Low setting 4 to 6 hours.
To assemble sandwiches, spoon about 1/3 cup beef mixture onto bottom halves of buns. Top each with scant 2 tablespoons cheese. Cover with top halves of buns.