In a 6-quart slow cooker, stir together the soup, 1 cup broth and onion soup mix until combined. Add the cubed steaks. Cook on high 3 hours or low 6 hours.
In a small bowl, whisk together the remaining 1/2 cup broth and cornstarch until dissolved. Stir in the cornstarch mixture; cook until the sauce is thickened, 30-35 minutes longer. Serve over egg noodles or mashed potatoes, if desired.