Cut the chicken lengthwise down the middle of the back, rinse it well, pat it dry, place it breast-side up on aluminum foil, and press it flat.
Whisk together the lemon juice, olive oil, salt, and pepper. Cut the sage leaves into fine strips and stir them into the marinade with the crushed chilli.
Brush the chicken all over with the marinade, wrap it in aluminum foil, and refrigerate for 2 hours.
Preheat the oven to 200°C (400°F).
Place the chicken on a baking sheet, open the foil packet, and fold the foil back so the marinade drains out.
Roast for 1 hour 30 minutes until the skin is crispy and the internal temperature reaches 74°C (165°F) in the thickest part of the thigh.