If turkey is not brined, rub bird inside and out with salt
Season inside and out with pepper and place on rack in pan
Place onion and celery in cavity
Rub 3 tablespoons butter over turkey
Roast turkey, uncovered, for 30 minutes
Meanwhile, stir remaining 3 tablespoons butter, soy sauce, and mirin (or substitute store-bought teriyaki sauce for soy sauce and mirin) in a small saucepan over medium heat until melted and smooth
Add rosemary
Cover; keep glaze warm over lowest heat
Reduce oven to 325°F
Baste turkey with pan juices; add more water if needed to maintain at least 1/4" liquid in pan
Roast for 30 minutes; baste with pan juices
Brush lightly with glaze
Continue roasting turkey, basting with pan juices and brushing with glaze every 30 minutes, and tenting with foil if turning too dark, until an instant-read thermometer inserted into thickest part of the thigh without touching bone registers 165°F (juices should run clear when thermometer is removed), about 2 3/4 hours total
Transfer turkey to a platter
Tent with foil; let rest for 1 hour before carving