Preheat oven to 350°F and grease a 9x13-inch casserole dish.
Add the cabbage and onion to your casserole dish.
In a small mixing bowl, stir together the cream of chicken soup, mayonnaise, salt, pepper, and garlic powder. Spoon the mixture over the cabbage.
Combine the Ritz crackers and melted butter in a medium mixing bowl. Sprinkle the cracker mixture over the casserole.
Bake uncovered for 35 minutes. (Tent with foil toward the last 10 minutes if you notice it over-browning.)
Sprinkle the cheese evenly over the top. Bake for an additional 5 to 10 minutes until the cheese is melted, and the cabbage is tender.
