Place steak on work surface; sprinkle with Italian seasoning, salt and pepper. Top with onion slice halves. Roll up beef, jelly-roll fashion, with onion inside; tie with string.
In 2- to 3-quart slow cooker, place beef roll, seam side down. Sprinkle garlic, mushrooms and tomato around roll. Spoon gravy over roll.
Cover; cook on Low setting 5 to 6 hours.
Remove beef roll from slow cooker; place on serving platter. Remove string; cut roll into slices. Stir gravy mixture in slow cooker until blended; pour into serving bowl. Serve beef roll slices with gravy.