Put the passata, lime juice, nutmeg, salt and chilli powder in a bowl or jug and stir well, adding the grappa or vodka, then chill in the fridge for at least 1 hr. Chill 6 cocktail glasses - small Martini glasses are best - at the same time.
Take two saucers. Pour a little water into one and sprinkle a couple of tbsp of sugar in the other. Take each glass and dip the rim into the water then the sugar. Divide the aperitivo between the glasses and decorate with slices of lime. Serve immediately.