Remove the sack of chicken innards from inside the chicken and discard.
Rub the inside of the chicken with garlic and Kosher salt.
Stuff the onion wedges, rosemary and thyme sprigs into the chicken cavity.
Place the chicken in the slow cooker.
Rub the olive oil over the outside of the chicken.
Combine the poultry seasoning and seasoning salt and sprinkle all over the outside of the chicken.
Cover and cook on LOW for 4 to 6 hours, or until the meat is tender, the drumsticks move freely when you wiggle them, and the juices run clear when pierced with a sharp knife.
Lift the chicken out of the slow cooker and onto a platter. Cover with foil to keep warm.
If desired, make a gravy right in the slow cooker: Stir together ¼ cup cornstarch and ¼ cup cold water until smooth. Pour mixture into the juices left in the slow cooker. Stir well. Cover and cook on high for about 10 to 15 minutes until thickened and bubbly. If the mixture gets too thick, thin to desired consistency with more water. Taste and add more salt and pepper if desired.