Combine the garlic, herbs, and salt in a small bowl.
Rinse the chicken under cool water and pat it dry with a paper towel.
Coat the outside of the chicken on all sides with the olive oil.
Rub the herb mixture over the inside and outside of the chicken.
Place the onion in the bottom of the slow cooker and lay the chicken breast-side down on top.
Place the lid on top and cook on low for 7-9 hours or until a meat thermometer reads 165 degrees in the thickest part of the thigh.
When the chicken is done cooking, place on a broiler pan or large cast iron skillet and broil in the oven for 4-5 minutes or until the skin is crisp if desired.
Slice chicken and serve.