Prepare the chicken. Arrange the boneless, skinless chicken thighs across the bottom of the slow cooker in an even layer.
Coat with starch. Sprinkle the tapioca starch or cornstarch uniformly over the chicken, lightly covering each thigh. The starch will help give the sauce a thicker consistency during cooking.
Add the liquids. Pour the salsa verde and chicken stock over the coated chicken. Position the chicken so that it is mostly covered by the liquid.
Slow-cook until tender. Place the lid securely on the slow cooker. Cook on high for 3–4 hours or on low for 6–8 hours, until the chicken is fully cooked, tender, and easy to pull apart.
Shred the chicken. Carefully transfer the cooked chicken from the slow cooker to a cutting board or large plate. Using two forks, pull the meat apart into bite-sized shreds.
Return to the sauce. Place the shredded chicken back into the slow cooker and stir thoroughly so the meat is evenly coated with the salsa verde sauce.
Allow the flavors to absorb. Leave the shredded chicken in the sauce on the warm setting for several additional minutes, allowing it to soak up more of the cooking liquid and flavor.
Serve. Spoon the salsa verde chicken onto plates while hot. Serve with rice, crushed tortilla chips, shredded cheese, sour cream, or other preferred toppings.