Preheat the oven to 350 degrees Fahrenheit.
Season the chicken breast with taco seasoning. Place it on a baking sheet and bake for 25-30 minutes, or until cooked through. Once cooked, remove from the oven and shred the chicken.
In a mixing bowl, combine the shredded chicken, diced onion, sour cream, and 1 cup of shredded cheddar cheese.
Fill each tortilla with the chicken mixture, roll them up, and place them seam side down in a 13 x 9-inch baking dish coated with a small amount of enchilada sauce.
Pour the remaining enchilada sauce over the filled tortillas in the baking dish, ensuring they are evenly covered.
Cover the baking dish with foil and bake for about 35 minutes, or until the enchiladas are bubbly and heated through.
Remove the foil and sprinkle the remaining shredded cheddar cheese over the top of the enchiladas. Return the baking dish to the oven and bake for an additional 5 minutes, or until the cheese is melted and bubbly.
Allow the enchiladas to cool for 5 minutes before serving.