Pat the whole chicken dry with paper towels and place it in the slow cooker.
In a small bowl, mix together the olive oil, salt, black pepper, garlic powder, onion powder, thyme, rosemary, and paprika to create a seasoning rub.
Rub the seasoning mixture all over the chicken, making sure to get under the skin where possible for maximum flavor.
Squeeze the juice of one lemon half over the chicken and place both lemon halves inside the cavity along with the smashed garlic cloves and quartered onion.
Cover the slow cooker and cook on low for 6-8 hours or on high for 4-5 hours, until the chicken reaches an internal temperature of 165°F.
Once cooked, carefully remove the chicken from the slow cooker and let it rest for 10-15 minutes before carving. Optionally, garnish with fresh herbs before serving.