Preparation:
The day before serving, make the gazpacho mix. Peel the onion and core and deseed the red pepper. Roughly chop the onion and pepper with the tomatoes and cucumber, and combine in a bowl with the vinegar, sugar, salt and a splash of olive oil. Cover and leave to marinate in the fridge for 24 hours.
The next day, transfer the marinated gazpacho mix to a blender and blend well for 2 minutes. Line a colander with a double layer of wet cheesecloth or muslin (or wet tea towel) and place the colander over a bowl. Pour the blended mix into the lined colander and allow it to strain into the bowl for at least 2 hours. (A wet cloth will not absorb the gazpacho but will allow it to pass through naturally without any force.)
Once the gazpacho has drained, soak the gelatine leaves in a bowl of cold water until they are soft, about 10 minutes. Warm 100ml of the gazpacho liquid in a small pan on top of the stove. Lift out and squeeze the softened gelatine, then add to the warm liquid and stir until dissolved. Pour back into the cold gazpacho and mix well.
Pour the gazpacho into a siphon canister (the same contraption used to whip cream in high-street coffee shops), s***w the lid on and place the canister in the fridge. Leave for at least an hour to allow the gelatine to set.
Method
Cut the tuna steak through the flesh against the grain into 4 slices. Place each slice between 2 sheets of clingfilm and gently bat the tuna out with a rolling pin to a disc that has an even thickness of about 2mm and is almost transparent.
Pick over the crab to ensure that there are no pieces of shell, then dry the meat well. Pick the leaves from the tarragon and finely chop them. Finely chop the chives.
Mix the crab with the chopped herbs, crème fraîche and seasoning.
Unwrap the tuna and lay each slice on a fresh piece of clingfilm that is four times larger than the slice. Brush the tuna with lemon oil and season with sea salt (this is part of the curing process), then place one-quarter of the crab mix in the middle of each slice. Bring the clingfilm up and around each piece of tuna to make a tight ball, then twist the film at the top and tie a knot in it. Now chill the tuna balls in the fridge for 1 hour to allow the shape to set.
Unwrap the tuna and discard the clingfilm. Brush the tuna with lemon oil and season with sea salt, then leave to stand until slightly warmer than fridge temperature.
Pick over, wash and dry the shoots, and have them ready to place on the plates.
Check that the lid of the canister is s***wed on tightly. Charge it with 2 gas cartridges and shake well, then gently squeeze the trigger to form a small ball of foam in the centre of 4 plates. Place a tuna ball alongside each, and scatter the shoots around to add colour, texture and flavour. Drizzle with lemon oil and serve.
Tip
We use a siphon in the restaurant, but at home you can equally well set the gazpacho in 4 bowls, allowing 2 hours for it to set as a jelly. To serve, place a tuna ball on top of each bowl of jelly and garnish as above.