Cut the passion fruit in half. Remove the flesh and put it through a sieve into a glass jug. Mix with nutmeg. Wash the limes with hot water, rub dry and cut into slices.
Put 2-3 ice cubes and a slice of lime in each of four long drink glasses. Distribute the sieved fruit juice into the four glasses. Top up with passion fruit juice and well chilled mineral water. Add a drinking straw and garnish with some lemon balm if desired.