Cook beef in 10-inch skillet over medium heat 8 to 10 minutes, stirring occasionally, until brown; drain.
Mix beef, tomatoes, soup, seasoning mix (dry) and water in 3 1/2- to 5-quart slow cooker. Gently stir in tortilla strips.
Cover and cook on Low heat setting 7 to 8 hours.
Sprinkle cheese over casserole; cover and let stand about 5 minutes or until cheese is melted. Sprinkle with onions; serve with sour cream.