Preheat oven to 350°F. Cook sausage and onion in large skillet over MEDIUM-HIGH heat 8-10 minutes or until sausage is thoroughly cooked, stirring frequently; drain. Return sausage mixture to skillet.
Add tomatoes, beans, water and seasoning mix; stir. Bring to a boil. Reduce heat to MEDIUM-LOW; simmer 10 minutes or until slightly thickened, stirring occasionally.
Place broken tortilla chips in lightly greased 13x9-inch baking dish; cover with layers of 1 cup cheese, sausage mixture and remaining cheese.
Bake 30 minutes. Cut into 8 squares. Serve topped with sour cream and cilantro, if desired.