Combine the chicken, salsa, onion, cumin, oregano, and salt in a saucepan. Simmer about 5 minutes or until most of the liquid is evaporated.
Brush one side of a tortilla with butter. Flip the tortilla over and spoon about 1/3 cup of the mixture onto the tortilla. Top with cheese. Fold sides over and place seam side down into a 13x9 baking dish sprayed with non-stick cooking spray. Repeat with the rest of the tortillas.
Brush any remaining butter on the tortillas - I usually use a little more than the recipe calls for because we like them really crispy!
Bake at 475 degrees anywhere from 15-25 minutes. The time will depend on how much butter you have used and how crispy you want your shells to get. Bake until they are crispy and golden.