Place the chicken breasts at the bottom of your slow cooker. Sprinkle the taco seasoning evenly over the chicken, then add the diced onion, diced tomatoes with green chiles, and chicken broth.

Cover and cook on LOW for 4 to 5 hours, or on HIGH for 2 to 3 hours. The chicken is ready when it is very tender and easily falls apart.

Use two forks to shred the chicken directly inside the slow cooker. Stir the shredded meat well so it absorbs the flavorful juices and let it sit on the warm setting for 10 minutes.

Preheat your oven to 400 degrees Fahrenheit. Arrange the hard corn taco shells standing upright in a 9x13-inch baking dish, fitting them snugly together so they do not tip over.

Spoon the shredded chicken mixture evenly into each taco shell, being careful not to overfill. Sprinkle the shredded cheddar cheese generously over the top of the chicken in each shell.

Bake in the preheated oven for 8 to 10 minutes, or until the cheese is completely melted and the edges of the taco shells are golden and crispy. Serve immediately with a garnish of fresh cilantro and your favorite toppings.