Lightly spray the crock with cooking spray.Season chicken breasts with salt and pepper and place chicken in the crock pot.Add salsa verde, chopped tomatoes, cumin, and chili powder; stir well to combine.Cover and cook for 3.5 to 4 hours on HIGH, or 5 to 6 hours on LOW.Remove cover and transfer chicken breasts to a cutting board; using two forks, shred the chicken.Stir the shredded chicken back into the crock pot.Taste for seasonings and adjust accordingly.Top with diced onions and fresh parsley or cilantro.Serve.