Remove stem and ovaries of tomatoes and add together with chipotle chilis to high speed blender (if you don't have a high speed blender you might have to quarter the tomatoes first) and blend until smooth.
Pour sauce into crock pot, add chopped onion and crushed garlic, give a quick stir and then submerge chicken breasts in the sauce.
Set slow-cooker to 3-4 hours on high or 6-8 hours on low.
Remove chicken breast from crock pot and place on a large chopping board and shred with two forks. Then add shredded chicken back into the crockpot and stir into the sauce.
Now build your tacos. Place a warm corn tortilla on a plate, add chicken tinga, lettuce, thinly sliced radish, cilantro leaves, sour cream and sliced avocado on top and enjoy!