Place chicken breasts into a gallon zip-lock bag.
Slice onion thinly and add it to the bag as well.
Add tequila, agave nectar, juice from the lime, salt, and chipotle chili powder to the bag.
Close the bag and gently shake the content to mix it.
Place the bag in a shallow bowl and into the fridge overnight. (You can also prepare this chicken in the morning and that way it will be ready to cook for dinner.)
Once chicken is marinated, preheat a medium pan over medium heat. Add a little bit of canola oil t the pan to cook the chicken.
Take chicken out of the marinade (SAVE the marinade, don't discard it) and cook it until done. (Cooking time will depend of chicken breast size.)
About half way through cooking the chicken, take onions out of the marinade and add them to the pan.
Once chicken and onions are done, take both out of the pan.
Add marinate into the hot pan and continue simmering it, stirring often, until reduced and thickened. Take sauce out and save it to use on tacos.
Slice chicken thinly. Pit and slice avocados.
Divide lettuce, avocado slices, cooked onions and chicken among the 4 tortillas.
Drizzle some sauce over the top and enjoy!