Spray the slow cooker/Crockpot with nonstick cooking spray (I also use a liner for easier clean-up).
Arrange the chicken breasts in an even layer in the slow cooker.
Sprinkle the diced onion evenly over the top of the chicken.
Pour all of the tomatillo salsa over the chicken and onions.
Secure the lid onto the slow cooker and set it to cook on low for 6 to 8 hours.
Remove the chicken from the slow cooker and shred it into bite-sized pieces using two forks.
Add the chicken back into the slow cooker and stir to coat the chicken in the sauce. The slow cooker should now be set to warm.
When ready to eat, add chicken to a tortilla or taco shell and top with your favorite taco toppings.
Notes
If you use a different tomatillo salsa/salsa verde, be sure to taste your cooked chicken after you shred it and mix it with the sauce. It may need additional seasoning before serving.