Pour one jar (or half) of the salsa verde on the bottom of the slow cooker.
Place chicken breast in the slow cooker and top with onion slices.
Pour remaining salsa verde over chicken breast.
Cook on high for 4 hours or low for 6 to 8 hours.
Notes
If you want some pieces with "browning," then you should minimize the amount you move the chicken while it's cooking. Just let it do its thing without stirring it or otherwise disturbing it.
For chunks of chicken vs. shredding: cook it long enough that you can break the chicken up with tongs instead of shredding it.