Put the ripe tomatoes into a pot of boiling water for 30 seconds and remove with a slotted spoon. When they have cooled a little, remove the skins. Quarter the tomatoes and remove the seeds and put the flesh in a blender with salt, sugar, olive oil and the basil and blend to make the granita mixture. Taste, and adjust flavours and seasoning if required. Spread the mixture evenly on a tray and freeze for couple hours. Use a fork to scrape off the frozen tomatoes and put this granita into a shot glass and keep in the freezer until service.
Next marinate the prawns. Remove the shells and put them in a bowl with olive oil, the chili powder, chopped coriander, lemon juice and seasoning and leave for 1-2 hours.
To make the purée, sauté the shallots in the butter until they are soft and tender. Pour in the chicken stock and add the watercress. Bring to just under the boil and reduce the heat and simmer everything for 20 minutes and then take off the heat and leave to cool. Place the cooled watercress in a food processor and add seasoning to taste and the crème fraiche and blend until the purée in smooth.
To make the crisps, sprinkle grated Parmesan evenly on a baking sheet and cook for 2-3 minutes in a medium hot oven. When it has melted and is golden it is done. Use a metal ring to cut out even sized circles of crisps and leave to cool.
Remove the prawns from their marinade and sauté them in butter in a hot pan. Add the chopped garlic and season. Add a splash of wine, the coriander and remove the prawns from the heat, before they are overcooked. Sprinkle with some lemon juice before service.
Put some watercress on a plate and place three prawns beside it. Put the shot glass of tomato granita on the side and balance the Parmesan crisp alongside it. Garnish with lemons and dressed baby salad leaves.