Blend the sauce. In a blender, combine tomatoes, chipotle peppers, adobo sauce, garlic, oregano, cumin, salt, pepper, vinegar, and chicken broth.
Blend until smooth. Taste and adjust heat by adding more adobo if you like it spicier.
Layer the slow cooker. Scatter the sliced onions in the bottom of the slow cooker. Place chicken thighs on top of the onions.
Pour the blended sauce over the chicken and tuck in the bay leaf.
Cook low and slow. Cover and cook on Low for 6–7 hours or on High for 3–4 hours, until the chicken is tender and shreds easily with two forks.
Shred and simmer. Remove the bay leaf. Shred the chicken directly in the slow cooker using two forks. Stir to coat the meat in sauce and let it simmer on Warm for 10–15 minutes to soak up the flavors.
Brighten and balance. Stir in the lime juice and taste.
Add more salt or a splash of vinegar if it needs extra punch. If the sauce is too thick, loosen with a bit more broth. If it’s thin, remove the lid and let it reduce for 10–15 minutes.
Warm the tortillas. Heat tortillas in a dry skillet, over a gas flame, or wrapped in foil in a low oven until pliable.
Keep them covered so they stay soft.
Assemble the tacos. Fill each tortilla with chicken tinga and your favorite toppings. Finish with a squeeze of lime.