Preheat oven to 400°F and arrange taco shells upright in a baking dish.
In a bowl, mix shredded chicken, enchilada sauce, cumin, garlic powder, chili powder, salt, and pepper.
Spoon chicken mixture evenly into each taco shell.
Top each taco with a mix of cheddar and Monterey Jack cheese.
Sprinkle red onions over the cheese.
Bake for 10-12 minutes or until cheese is melted and shells are crispy.
Garnish with fresh cilantro and a dollop of sour cream if desired.