Heat oven to 350°F.
Combine chicken, chiles, onion and cheese in a bowl.
Heat a skillet over low heat. Pour a small amount of oil in the bottom. Heat each tortilla in oil for about 20 seconds per side or until pliable and warm but not crunchy. Place tortillas on a plate with paper towels to drain.
Lightly oil a 9-inch baking dish. Spread a thin layer of of sauce in the bottom.
Assemble enchiladas. Spoon about ¼ cup chicken mixture into a tortilla and roll up. Place in the baking dish, seam side down. Repeat with remaining tortillas and chicken.
Pour enchilada sauce over enchiladas, spreading with a spoon to cover tortillas entirely. Sprinkle with remaining cheese.
Bake enchiladas 15-20 minutes or until sauce is bubbly and cheese is melted. Remove from oven and serve with cilantro, jalapeño slices, diced red onion, or avocado (optional).