Preheat oven to 350 degrees.
Pour the enchilada sauce into a medium saucepan and heat over low to warm.
In a large skillet, heat the oil over medium heat. Add the onion, green pepper and, if using, the minced jalapeño. Add the cooked chicken, chile powder, cumin, salt and black pepper to taste. Stir in about 1/2 cup of the warm sauce and 1/2 cup cheese. Stir to combine. Spread about 1/4 cup of the enchilada sauce in the bottom of a 9- by 13-inch baking dish.
Working with several corn tortillas at a time, place them on a plate and microwave about 45 seconds to soften.
Spoon about 1 tablespoon of the sauce on a warmed tortilla, spreading it around. Put about 1/4 cup of the chicken mixture on the tortilla and roll up. Place in the baking dish. Repeat with the remaining tortillas, fitting them snugly in the baking dish.