In a medium bowl, mix the chicken, cheese, salsa, chili powder, cumin, and green onion. Make ahead tip: The chicken mixture can be prepared, covered, and refrigerated up to a day in advance.
When ready to eat, preheat the oven to 400 degrees F. Place about 1/3 cup of the chicken mixture in the center of each tortilla. (I lay all the tortillas out on the counter so I can adjust the filling and make them all even. I also like to form the filling into a rectangle for even distribution once rolled.) Fold opposite sides over filling—I fold across the short end of the filling first. Fold in the remaining two sides to form a little package, and place seam-side down on a baking sheet. (No need to grease; may line with foil or parchment for really easy cleanup.) Brush the tops and sides of the chimichangas with the melted butter or oil. Bake for 20 minutes, give or take a few minutes, or until golden brown on top.
Serve with toppings of choice. (Be careful if biting in immediately. The filling will hot!)