Move oven rack one level below center and preheat oven to 350 degrees.
In a small mixing bowl, stir together butter, garlic, thyme and season with a little salt and pepper.
Remove neck and giblets from turkey and remove any pin feathers left behind. Tuck wings under turkey. Season cavity of turkey with salt and pepper.
Using the back end of a wooden spoon, separate turkey breast skin from breast. Rub butter underneath skin, smooth over skin to spread butter out.
Season all over turkey with a fair amount of salt and pepper.
Add 1 tablespoon flour to the Reynolds Kitchens® Turkey Oven Bag and shake. Place oven bag in a roasting pan, add celery, onion and carrots to oven bag. Place turkey over vegetable layer in oven bag.
Close oven bag with the oven safe tie included. Cut 6 slits across top of the oven bag to allow steam to escape, then tuck ends of oven bag into the pan.
Cook unstuffed turkey according to size (thickest portion of thigh and breast should register 165 degrees in several places on an instant read thermometer):
12–16-lb. turkey: 2–2 1/4 hours
16–20-lb. turkey: 2 1/4–2 1/2 hours
20–24-lb. turkey: 2 1/2–3 hours
Let turkey rest 20 minutes. Carefully cut top of oven bag open, transfer turkey to carving board and carve.