Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13-inch baking dish.
Melt butter in a small skillet over medium heat. Add onion and garlic; cook and stir until tender, 4 to 6 minutes (see Cook's Note). Remove from heat and set aside. In a large bowl, combine corn, cream-style corn, salt, and egg substitute. Beat in muffin mix. Fold in cooked onion mixture. Pour into prepared baking dish.
In a large bowl, combine chicken, green chiles, mushrooms, sour cream, salt and pepper. Spoon over corn mixture to within 1 inch from edge. Sprinkle top with cheese.
Bake in preheated oven for 35 to 40 minutes, or until edges are golden brown.