Process the onion, garlic, lemon peel, chilli flakes, paprika, oil and vinegar in a blender to a smooth paste.
Use poultry scissors to cut the chicken along the backbone, turn over and press flat. Cut into the meat with a sharp knife, then brush with the seasoning paste, cover and refrigerate overnight.
Preheat the oven to 200 ° Place the chicken skin-side up in an oven-safe dish. Roast in the oven for 40-50 minutes until cooked.
Sprinkle with chopped parsley and drizzle with gravy. Serve with lemon wedges.