Put 4 pieces of string, about 30 cm (12 inches) long, into cold water to soak for 10 minutes.
Put the oil, lemon juice, chopped dill, spring onions, mustard powder and salt and pepper in a bowl and mix well. Wash and dry the vine leaves and arrange them in pairs, overlapping them slightly.
Make several slashes on both sides of each fish and rub them all over with a little of the oil and lemon mixture. Stuff each of the belly cavities with a bay leaf and a dill sprig. Lay each fish on a pair of vine leaves and wrap securely. Brush with a little of the oil and lemon mixture and fasten with the wet string to secure the leaves in place.
Cook the fish on a hot barbecue for 4—5 minutes on each side, brushing them with a little more of the oil and lemon mixture if necessary, until lightly charred. Leave the fish to rest for a few minutes, then discard the string and vine leaves and dress the mullet with the rest of the oil and lemon mixture. Garnish with dill sprigs and serve with a Tomato & Olive Salad (see