Preheat the oven to 200C/400F/Gas 6.
Place the reserved zested and juiced lemon pieces into the fish's stomach cavity. Score the skin with a sharp knife.
Mix together the oil, chilli, garlic, parsley and lemon zest in a bowl, then rub the mixture into the skin of the fish.
Place the egg whites and salt into a separate bowl and mix well.
Line a baking tray with greaseproof paper and place one third of the salt and egg white mixture on it.
Place the fish on top of the layer of salt, then spoon the remaining salt and egg white mixture over the fish to completely cover.
Place into the oven to bake for 35 minutes, or until the fish is just cooked through.
To serve, peel off the hard baked salt crust from the fish, then spoon over the lemon juice.
Serve with roasted vegetables and roasted chestnuts.