Preheat oven to 200°C/180°C fan-forced.
Place potatoes in large saucepan. Cover with cold water. Bring to the boil over high heat. Boil for 5 minutes. Drain.
Remove leaves from 1 bunch of sage and finely chop (you need 2 tablespoons). Place garlic, chopped sage and 250g butter in a bowl. Season with salt and pepper as required. Mash with a fork until combined. Using your fingers, gently loosen turkey skin at narrow end of breast. Gently push butter mixture between skin and breast. Rub skin to distribute butter mixture. Secure skin with toothpicks. Place in a large, flameproof roasting pan. Place potatoes around turkey. Season with salt and pepper as required.
Roast for 40 minutes, basting turkey and potatoes with pan juices twice during cooking. Add onion to pan. Roast for a further 30 minutes or until juices run clear when thickest part of turkey is pierced with a skewer.
Transfer potatoes to a serving bowl. Cover with foil to keep warm. Transfer turkey and onion to a plate. Cover loosely with foil. Stand for 15 minutes.
Meanwhile, remove leaves from remaining bunch of sage (you’ll need 1/3 cup). Skim fat from pan juices. Place roasting pan over medium heat. Add remaining butter. Cook until melted and foaming. Add eschalot. Cook for 2 minutes or until softened. Add sage and hazelnuts. Cook for a further 4 to 5 minutes or until sage is crisp.
Slice turkey. Place turkey and onion on a serving plate. Spoon over hazelnut and sage butter sauce. Serve with potatoes and beans.