Lay the chicken in the bottom of the slow cooker. Cover with diced onions, garlic, oregano. cumin, and chili powder.
Remove the chipotle peppers from the can. Split them open with a knife and scrape the seeds out. Then add the tomato sauce, chipotle peppers and remaining sauce in the chipotle peppers can to the slow cooker.
Cook on high for 2.5-3.5 hours.
Once done, remove the chicken from the crockpot. Optional: Use an immersion blender to blend the sauce until smooth.
Shred the chicken, and then stir it back into the sauce.
To assemble tacos:
Heat both sides of each tortilla over high heat until it just starts to brown.
Fill each tortilla with meat. Add lettuce, pico de gallo, and a sprinkle of cotija cheese. Serve immediately.