Preheat your oven to 375 degrees Fahrenheit. Grease a 9x13-inch baking dish with a little oil to prevent sticking.
Prepare the filling: In a large bowl, combine the shredded chicken, chopped onion, bell pepper (if using), green chilies (if using), garlic powder, cumin, salt, and pepper. Add half a can of enchilada sauce. Mix well until everything is evenly distributed.
Soften the tortillas: To make the tortillas pliable and easy to roll, heat them briefly on a skillet over medium heat for about 30 seconds on each side or until they are warm and soft. Alternatively, you can wrap them in a damp cloth and microwave them for about 30 seconds.
Assemble the enchiladas: Pour a little enchilada sauce in the bottom of the greased baking dish—just enough to coat the surface. Take a tortilla, place a generous amount of the chicken mixture down the middle, and sprinkle with some of the cheese. Roll the tortilla tightly around the filling and place it seam side down in the baking dish. Repeat with the remaining tortillas and filling.
Top and bake: Once all the enchiladas are in the dish, pour the remaining enchilada sauce over the top, ensuring all the tortillas are covered. Sprinkle the remaining cheese over the top.
Bake for about 20-25 minutes, or until the cheese is melted and bubbly and the edges of the tortillas begin to get a slight golden crust.
Garnish and serve: Remove from the oven and let sit for a few minutes before serving. Garnish with chopped fresh cilantro and serve with a dollop of sour cream on the side.
