Preheat An Oven To 350 Degrees F (175 Degrees C). Fill A Large Pot With Lightly Salted Water And Bring To A Rolling Boil Over High Heat. Once The Water Is Boiling, Stir In The Rotini, And Return To A Boil. Cook Uncovered, Stirring Occasionally, Until The Pasta Has Cooked Through, But Is Still Firm To The Bite, About 8 Minutes. Drain Well In A Colander Set In The Sink. Place The Cooked Pasta Into A Large Mixing Bowl, And Add The Cream Of Mushroom Soup, Ketchup, Green Chiles, Onion, And Luncheon Meat. Pour Into A 9X13 Inch Baking Dish, And Evenly Sprinkle With The Cheddar Cheese. Bake In The Preheated Oven Until The Casserole Is Hot And The Cheddar Cheese Has Melted, About 10 Minutes.