Pat chicken dry and season all over with salt and pepper.
Secure legs together and tuck wing tips under the body of the chicken.
Preheat oven to 425°.
Fill a 9"-x-13" baking dish with chopped onions, carrots, celery, potatoes, and garlic and toss together with oil and 1 teaspoon salt.
Place chicken on top.
Brush chicken all over with melted butter and stuff cavity with lemon and most of the thyme, reserving a couple sprigs for garnish.
Roast until juices run clear and a meat thermometer inserted into the center of the thigh reads 165°, 50 minutes to 1 hour depending on weight of chicken.
Tent chicken with foil and let rest 15 to 20 minutes, then transfer to a cutting board and carve.