Place onion, celery, and carrot in a large, shallow roasting tin.
Place turkey, breast side up, on top of the vegetables in the roasting tin. Pat the outside and inside of the turkey dry with kitchen paper.
Combine salt, black pepper and cayenne pepper in a small bowl. Season the inside of the turkey with about 1/3 of the salt mixture. Fold wing tips under the bird.
Melt butter in small saucepan over medium heat until the edges begin to turn golden, about 2 minutes. Cook and stir rosemary and sage for 1 minute.
Place rosemary and sage inside the cavity of the turkey; reserve melted butter. Tie the legs together with kitchen string.
Brush outside of the turkey completely with the melted butter. Season with remaining 2/3 of the salt mixture.
Bake turkey, uncovered, in the preheated oven until no longer pink at the bone and the juices run clear, about 3 hours. A kitchen thermometer inserted into the thickest part of the thigh, near the bone should read 82 degrees C. Remove the turkey from the oven and allow to rest in a warm area (covered with a double sheet of aluminium foil) 10 to 15 minutes before carving and serving.