Place diced onion on bottom of slow cooker and turn on low.
Trim chicken breasts of excess fat and place on top of onions.
Place garlic and herbs on top of chicken.
Pour in 1/2 cup of chicken broth around the chicken.
Put lid on and cook for 5 hours. Depending on size of chicken breasts, you can check a little early with a meat thermometer. Chicken should register 165 degrees Fahrenheit in thickest section. Don’t cook for more than 6 hours – chicken will become dry and tough.
Freeze unused chicken in freezer bags for up to 3 months.