In a small bowl, combine the olive oil, oregano, thyme, sage, garlic powder, sea salt, and black pepper. Rub the mixture all over the chicken.

Place 2 or 3 lemon and onion pieces and thyme sprigs into the chicken cavity.

Place the chicken into a 4- to 6-quart slow cooker. Add the remaining onion, lemon, and thyme sprigs around the chicken.

Cover and cook on LOW for 8 hours. Serve topped with additional thyme leaves, if desired.