Heat oil in a large skillet, Season chicken with salt, pepper, and thyme.
Brown chicken in skillet, about 2-3 minutes on each side. Remove from skillet.
Add onion to the pan and cook for 2-3 minutes, until soft.
Place onions in the bottom of the slow cooker., add mushrooms and chicken.
Add broth and cook on low for 6-8 hours, depending on the size of your chicken breasts.
When the slow cooker is finished, remove the chicken and top with mushroom and onion sauce.