Saute the onion in a skillet until it begins to become translucent. Then, add the chicken and cook according to package directions or until heated through. Add half of the enchilada sauce and half the bag of cheddar cheese.
Grease a baking dish (you may need 2). Fill each tortilla with 2 spoonfuls of the chicken mixture. Roll and place crease-side down in the baking dish. Top with remaining enchilada sauce and both types of cheese.
Bake for 15-20 minutes or until the sauce bubbles and the edges are crispy.