Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
In 12-inch nonstick skillet, cook beef and onions over medium-high heat, stirring frequently, until brown; drain. Stir in seasoning mix and water. Heat to boiling. Reduce heat to medium; simmer uncovered 3 to 4 minutes or until thickened. Stir in enchilada sauce and black beans; cook 1 to 2 minutes or until heated through.
Place 4 cups of the corn chips in single layer in baking dish. Top with 1 cup of the cheese; spoon beef mixture over cheese, then top with remaining 1 cup cheese.
Bake 18 to 20 minutes or until cheese is melted and edges are bubbly. Top with remaining corn chips and the green onions. Serve with sour cream and guacamole.