Peel, core and cut your cucumber into very small dices (brunoise). Cook them for 30 seconds in some salted boiling water, plunge them strait away in ice-cold water, strain them and set aside.
Pan-fry the tails of crayfish in a little bit of olive oil for a couple of minutes. Then set aside.
Pre-heat your oven at 350ºF (gas mark6).
Put your egg yolks, whole eggs, 20 cc double cream and curcuma in a bowl. Season with salt and a bit of cayenne pepper. Then mix for a minute of so. Add in the crab meat, mix for another 10 seconds.
Take a shallow roasting tray, pour some boiling water, half way up. This is your bain-marie (double boiler) done up.
Place 5 tails of crayfish at the bottom of each dishes, then pour some of the egg-cream mixture on the top of them. Cook for 35 minutes. (check if it's cooked with tip of a knife).
While your flans are cooking, start putting your sauce together. In a pot bring the remaining cream to the boil and let it reduce a bit. Add in the fish stock and bring to the boil again. Just before serving, add the curry and check the seasoning. Keep hot in a double boiler.
If you have, use 8 nice (black or white) plates to be able to see the contrast of colors. In the center of each plates, "un-mold" your flan. Put some of the cucumber around it and coat with some curry sauce. Place a sprig of chervil on the top of the flan. If you have used fresh crayfish to realise this dish, place the head of one of them on the side of each plates. If you have prepared this recipe in advance, heat-up your flan in the oven at 285F for 7 to 8 minutes. Put the sauce on the range on low heat.