If beef roast comes in netting or is tied, remove netting or strings. In 4- to 5-quart slow cooker, place onions. Place beef on onions. In small bowl, mix diced tomatoes and tomato paste; pour over beef.
Cover; cook on Low heat setting 8 to 10 hours.
Remove beef from cooker; place on cutting board. Use 2 forks to pull beef into shreds. Return beef to cooker. Spoon about 1/2 cup beef mixture onto each bun. Top each with 1/4 cup cheese.