Peel 2 cucumbers and cut the skins into julienne strips. Blanch in boiling salted water for 5 minutes, then cool in ice water. Set aside.
Cut the third cucumber into olive shapes, retaining some skin for visual appeal. Blanch in boiling salted water for 3 minutes, then refresh in ice water. Set aside.
Gather the remaining cucumber flesh and blend until smooth. Strain through a fine sieve into a large sauté pan. Season the turbot fillets and poach gently for about 6 minutes per side, depending on thickness.
Drain the fillets with a slotted spoon and rest on a paper towel-lined plate.
Strain the cooking liquid again and add the cucumber julienne.
Arrange the fillets on 4 warmed plates, garnished with the cucumber olive shapes. Warm briefly in a low oven.
Incorporate the butter pieces into the sauce off heat, adjust seasoning, and stir in the chopped chervil.
Remove plates from oven and generously spoon the sauce over the fillets.
Expert Tip: Brill makes an excellent, more affordable substitute for turbot. Note that cucumber juice may separate into water and pulp when heated—don't worry; it emulsifies beautifully once the butter is added.
