HAZELNUT SEASONING: Process hazelnuts, sage, olives and garlic until combined. Add butter; pulse until mixture is smooth. Transfer to a medium bowl; season to taste.
Preheat the oven to 150°C/130°C fan forced. Line an oven tray with baking paper; coat with cooking-oil spray.
Rinse chicken inside and out with water. Pat dry cavity and skin with paper towel. Run your fingers carefully between the skin and the breast meat over the legs and thighs. Place seasoning under skin, pressing gently to spread evenly. Place lemon in the cavity; tuck wings under. Season.
Turn chicken, breast-side down. Place prosciutto across chicken in one layer, overlapping pieces to help it stay in place. Wrap a slice of prosciutto around each wing. Turn chicken over; continue placing prosciutto across the top (you may need to use two slices end to end for the width). Cover drumsticks with prosciutto; tie legs together with kitchen string.
Place chicken, breast-side up, on tray; coat chicken with cooking-oil spray. Roast chicken for 1½ hours or until cooked through. Remove chicken from oven, cover loosely with foil; rest for 15 minutes.
Serve chicken with rosemary.
Use poultry shears or sharp kitchen scissors to cut raw or roasted chicken easily and safely.
To achieve a delicious crispy skin, make sure the surface of the chicken is dried thoroughly. Basting the chicken with the pan juices during roasting also helps to brown the skin and keep the chicken moist.