Preheat the oven to 500 degrees F (260 degrees C). Grease a 9x13-inch baking pan.

Heat olive oil in a skillet over medium heat. Add onion and red bell pepper; cook until softened, 3 to 4 minutes. Set aside.

Combine chicken, onion-pepper mixture, Mexican cheese, chiles, BBQ sauce, and chili powder in a bowl.

Brush both sides of each tortilla with butter. Warm tortillas in a skillet over low heat, about 5 minutes.

Spoon 1/3 cup chicken mixture down the center of each tortilla. Roll up tightly; place seam-side down in the prepared baking pan.

Bake in the preheated oven, brushing with any remaining melted butter, until golden brown, 10 to 15 minutes.