Place all the ingredients with the exception of the chicken breasts in a food processor.
<g class="gr_ gr_90 gr-alert gr_spell gr_run_anim ContextualSpelling ins-del multiReplace" id="90" data-gr-id="90">Pureer</g> the ingredients until a thick, rich sauce is formed.
Place the whole chicken breasts in the bottom of your slow cooker and pour the sauce from the food processor on top, covering the chicken completely.
Cover your slow cooker and cook on low for 6-8 hours or high 3-4 hours.
Once cooked, turn off the crockpot and with the chicken still in your slow cooker, use a pair of metal tongs or two forks to shred the chicken, stirring it into the extra sauce.
As you shred and combine the chicken and sauce, you'll notice the chicken begins to soak up any additional sauce in the crockpot.
To serve, place shredded chicken and sauce on top of a tortilla and top with avocado, cheese, and a bit of onion.