Heat oil in a pan. Sauté onions 5–6 minutes until caramelized, then add garlic and cook 1 minute. Stir in chicken + 1 cup enchilada sauce. Remove from heat.
Preheat oven to 350°F. Spread ¾ cup enchilada sauce on the bottom of a 9x13 baking dish.
Fill each tortilla with chicken mixture + ¼ cup cheese. Roll tightly, seam side down in the dish. Top with ¾ cup enchilada sauce + 3/4 cup shredded cheese.
Bake 20 minutes or until tortillas are cooked and cheese is melted.
Top with lettuce, tomatoes, crema, and onion/cilantro if desired. Serve warm.