For the tomato paste:
Juice the tomatoes and remove any seeds.
Transfer the tomato juice to a saucepan and slowly cook down to 750g until a rich paste is achieved. Season with the tamarind paste and salt and refrigerate until needed. Any amount that is unused will store well in vacuum for around 2 weeks or can be frozen.
For the prawns:
Remove the heads and shells from the prawns and devein the meat. Keep the heads on absorbent paper, taking care not to break the legs or antennae – these will be grilled later.
You will need 75g prawn meat. Halve each prawn lengthways and cut the meat into an even small dice. Blanch the shallots in boiling water for around 10 seconds. Strain and, when cool, add to the prawn meat, along with the other ingredients. Check the seasoning and add lime zest and juice to balance the seasoning if necessary. Reserve, refrigerated, until service.
For the nasturtium:
Blanch the nasturtium leaves in salted water for approximately 5 seconds so they are just wilted. Refresh in iced water and lay them out to dry on absorbent paper.
For the vinaigrette:
Chop the garlic super-finely, then combine it with the remaining ingredients, but add the parsley just before serving or the acid in the vinegar will oxidize it and turn it an off-green to brown colour.
To finish:
Prepare a barbecue.
Fill each of the nasturtium leaves with 15g of the prawn mix, folding each leaf neatly into a parcel, and leaving the stem still attached to the leaf on the outside. Place a little of the finger lime on top, then arrange on a plate with even distances between the parcels and the rest of the plate. Dress the prawn heads in the vinaigrette and grill them quickly over charcoal, basting them with more vinaigrette as they cook. Cook them to medium-rare so that the juices inside do not coagulate but are slightly cooked and do not taste raw. Season the prawn heads with salt to taste and place them between the parcels on the plate. It should appear that each head belongs to a parcel (its tail).