Heat the oven to 375°F/190°C.
Sprinkle the chicken all over with salt, black pepper, granulated garlic, granulated onion, fresh thyme, and fresh rosemary.
Toss the remaining onion, celery, and carrot with most of the herb basting sauce, leaving a small portion aside for brushing later. Scatter these vegetables in the roasting pan.
Stuff the chicken cavity with some of the seasoned vegetables.
Sprinkle the exterior of the stuffed chicken with garlic powder, onion powder, and smoked paprika. Place the chicken on top of the vegetables in the pan.
Roast the chicken. After it has started cooking, brush occasionally with the reserved herb basting sauce to keep the skin moist, flavorful, and golden. Roast until the thickest part of the thigh reaches 160°F/70°C, about 60 to 80 minutes. Transfer to a cutting board and rest for 10 to 15 minutes.
Tilt the roasting pan and skim excess fat. Set the pan over medium heat, add water or stock, and scrape up the browned bits. Simmer until slightly reduced to make the pan au jus.
Boil the potatoes in salted water until very tender, 15 to 20 minutes. Drain thoroughly. Mash with butter and warm milk until smooth and creamy. Add more milk if needed and season to taste.
Carve the chicken and serve alongside the mashed potatoes, drizzling with warm pan jus.