Cut chicken breasts into strips. Spray the inside of the slow cooker with olive oil or non-stick spray and arrange the chicken strips in the bottom. Mix together the tomato salsa, onion powder, 4 ounce can diced green chilies with juice, 1-2 tablespoons hot sauce and 2 tablespoons lime juice. (Start with 1 tablespoon of hot sauce and taste the mixture on your finger to see if you want more.) Pour the sauce mixture over the chicken and cook on low for about 5 hours, or until chicken is very tender. (You can cook for a shorter time on high, but I thought cooking on low gave more tender chicken.)
When the chicken is done, remove each piece with a slotted spoon and lay it on a cutting board to cool. Shred chicken with a fork and return to the slow cooker, stir to combine with the sauce, and let it heat back up while you prepare other ingredients.
Peel and dice the avocados and toss with the 1/4 cup lime juice. Add the chopped cilantro and 1/4 cup chopped red onion and stir to combine.
For tacos, cut iceberg lettuce heads in half and wash in salad spinner if needed. Peel off two pieces of lettuce and fill with a scoop of chicken mixture and a scoop of avocado salsa. Eat tacos with your hands.