Start by rinsing the whole chicken under cold water and patting it dry with paper towels. Remove any giblets from the cavity.
In a small bowl, mix together the salt, black pepper, garlic powder, onion powder, thyme, rosemary, and paprika.
Rub the olive oil all over the chicken, then generously sprinkle the spice mixture over the chicken, making sure to coat it evenly, including inside the cavity.
Place the halved lemon, smashed garlic cloves, and quartered onion inside the cavity of the chicken.
Place the chicken breast-side up in the slow cooker. Cover and cook on low for 6 to 8 hours or on high for 4 to 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 about 10 minutes before carving. Serve warm, garnished with fresh herbs if desired.