In a bowl mix granulated garlic, granulated onion, paprika, ground thyme, cayenne pepper, dried sage, salt and black pepper in bowl.
Coat chicken with 1 ½ tbsp olive oil and 2/3 of seasoning mix. Cover chicken and let marinate for at least 2-4 hours.
Preheat oven to 400 degrees.
Chop potato and onion into large pieces then add to roasting pan. Pour remaining 1 ½ tbsp oil and seasoning over vegetables and mix to coat well.
Place chicken on top of the vegetables and add thyme sprigs. Place in preheated oven and roast for 45 mins to 1 hour until the chicken is golden brown, the vegetables are tender and the chicken has reached an internal temperature of 165 degrees. Remove from oven and serve.