Place chicken breast (1 pound) in a pot covered with 1" of water. Sprinkle water generously with salt and pepper.
Bring water to a boil, then reduce to a simmer. Cook the chicken like this for 15 minutes. Check to make sure the internal temp of the chicken has reached 165 F, then remove from the pot and set aside on a plate to cool.
Once cool enough to handle, shred the chicken using 2 forks.
Meanwhile, heat vegetable oil (2 tablespoons) in a large skillet and saute chopped onion (1/2 cup) until translucent, about 8 minutes.
Add Low Carb Taco Seasoning (4 tablespoons) and toast until fragrant, about 1-2 minutes.
Add tomato sauce (1/4 cup) and water (1/4 cup) and stir until the seasoning dissolves.
Add the shredded chicken and toss to coat in the sauce. Remove from the heat and set aside.
Meanwhile, preheat the oven to 400 F and line a baking sheet with parchment paper.
Create flat circles of cheese on the baking sheet, using cheddar cheese (1/4 cup) for each circle.
Bake in the preheated oven for 8-10 minutes. You'll know the cheese is done once it starts to lightly brown along the edges.
Remove baking sheet from the oven, and place chicken, lettuce, and tomatoes on one half of the cheese.
Once the cheese is cool enough to handle (but don't wait until the cheese has hardened, about 5 minutes, grab the corner and fold it over the toppings. The cheese will crisp as it cools.
Serve with salsa and enjoy!