Place the chicken breast in a large stockpot. Add enough cold clean water to cover it by approximately 2 to 3 inches. Put the top on the pot and bring the water to the boil over high heat. Reduce the heat to medium once water is boiling and cover the pot again. Cook the chicken for 12-15 minutes over medium heat until chicken is cooked through. Remove the chicken breasts and set on a plate to cool. When cool enough to touch, shred the chicken breasts with a fork and set aside.
In a small bowl, combine tomatoes, onion and garlic. Set aside.
Place the tortillas on two baking sheets and bake until slightly crispy, about 4-6 minutes.
Put ¼ cup of chicken and 2 Tbsp of shredded cheese on each tortilla. Place them back in the oven and broil the cheese until just melted, about 2-3 minutes.
Top each tortilla with a heaping ¼ cup of salsa, ⅛ of the avocado and the juice from one lime slice.