In A Large Resealable Plastic Bag, Combine The Cornmeal, Cornstarch, Garlic Salt And Cumin. In A Shallow Bowl, Beat The Egg And Water. Flatten Chicken To 1/4-In. Thickness; Dip Into Egg Mixture, Then Place In The Bag And Shake To Coat. In A Large Skillet, Heat Oil. Cook Chicken For 4 Minutes On Each Side. Top With Cheese And Avocado; Cover And Cook Until Chicken Juices Run Clear And Cheese Is Melted. Garnish With Sour Cream And Onion.