Step 1: Prepare the Flavor Foundation
If using onion, place the finely diced onion at the bottom of the crockpot insert. This allows it to soften and sweeten as it cooks, creating a flavorful base for the chicken. Pat the chicken breasts or thighs dry with a paper towel and place them in a single layer over the onions. Season the chicken lightly with a pinch of salt and pepper.

Step 2: Create the Cooking Sauce
In a medium bowl, combine the entire jar of salsa, the packet of taco seasoning, fresh lime juice, minced garlic, and the optional cumin and chili powder. Whisk these ingredients together until they are fully incorporated into a cohesive, fragrant sauce.

Step 3: Combine in the Crockpot
Pour the sauce mixture evenly over the chicken in the crockpot, ensuring each piece is coated. Use a spoon or spatula to gently shift the chicken around so some sauce gets underneath. There is no need to add extra water or broth; the salsa and chicken will create plenty of liquid as they cook.

Step 4: The Long, Slow Cook
Place the lid securely on the crockpot. Cook on LOW for 6-7 hours or on HIGH for 3-4 hours. The chicken is done when it is tender enough to be easily shredded with two forks. Cooking on low is always preferred for the most tender, pull-apart texture.

Step 5: Shred the Chicken
Once the chicken is fully cooked, carefully remove the lid. Using two forks, shred the chicken directly in the crockpot. One fork should hold the chicken steady while the other pulls the meat apart into thin strands. The chicken will absorb the surrounding liquid as you shred it.

Step 6: Incorporate and Rest
After shredding, stir the chicken thoroughly to coat every strand in the now-concentrated cooking liquid. For a saucier consistency, you can leave it as is. For a drier, more traditional taco filling, let it sit in the warm crockpot with the lid off for 15-20 minutes to allow some liquid to evaporate.

Step 7: Serve and Build Your Tacos
Warm your tortillas by wrapping them in a damp paper towel and microwaving for 30-45 seconds, or by charring them briefly over a gas flame. Set up a taco bar with the warm shredded chicken and all your favorite toppings, allowing everyone to build their own perfect taco creation.