Place your chicken and onion into the bottom of your slow cooker.
In a small mixing bowl, combine the garlic, chipotle in adobo, tomatoes, and birria seasoning. If you don't want it spicy, use only half the can of chiles.
Pour the sauce mixture over the chicken and stir.
Cover and cook on low for 6 hours or high for 4 hours.
Remove the chicken from the slow cooker and shred.
To assemble the tacos, heat a non-stick skillet over medium heat.
Dip a tortilla into the cooking liquid and place it into the heated pan.
Add some shredded chicken, cheese, and toppings to the tortilla.
Fold the tortilla over and flip to cook on the other side.
Remove and repeat with the rest of the tacos.
