Layer the base. Add the sliced onion and garlic to the bottom of the crockpot. Pour in the chicken broth and salsa. Stir in the tomato paste, cumin, and half the taco seasoning.
Season the chicken. Pat the chicken dry, then sprinkle with the remaining taco seasoning plus a pinch of salt and pepper.
Place the chicken on top of the salsa mixture.
Cook low and slow. Cover and cook on Low for 4–6 hours or High for 2.5–3.5 hours, until the chicken shreds easily with two forks. Thighs may finish a bit sooner than breasts.
Shred and brighten. Transfer the chicken to a cutting board and shred. Stir the lime zest and juice into the crockpot sauce, then return the shredded chicken and toss to coat.
Adjust salt and pepper as needed.
Warm the tortillas. Heat tortillas in a dry skillet for 30–45 seconds per side, or wrap in a damp paper towel and microwave for 30–45 seconds until pliable.
Assemble the tacos. Spoon the chicken into tortillas and add your favorite toppings. Finish with a squeeze of lime or a splash of hot sauce.