Melt butter in a Dutch oven over medium heat. Add onion; saute until tender. Add flour, garlic salt, and hot sauce; cook, stirring constantly, 1 minute. Gradually stir in milk; cook until thickened and bubbly. Reduce heat; stir in cheese until melted. Add ham and turkey; cook, stirring occasionally, until heated. (Do not boil.) Serve with desired toppings.