Make tamarind water; mix 1 to 4 in warm water and put through a sieve. Or use ready made tamarind water.
Add the lemonjuice, sugar, sesameoil, ginger, fishsauce, chili, kaffir lime leaves and sereh to the tamarind water en stir.
Chop peanuts finely.
Cut the cilantro and mint finely and mix the two together.
Peel the cucumber and cut into slices of about 0,3 cm thick.
Tuna filet (about 1 cm thick) quickly sear on both sides for about 45/50 seconds each side and season with salt and pepper.
Cut the tuna into cubes or rectangles of about 10 gram each and put them on the cucumber slices. Cover with some of the sauce, top with the peanuts and last with the cut herbs. Serve immediately.