Peel the cucumber thinly then cut in half lengthwise and scoop out the seeds.
Cut across into half-moons about 1 cm/1/2 inch wide.
Blanch in boiling water for about 4 minutes, but do not let the colour start to fade.
Drain and put into cold running water, which turns the cucumber a delicious translucent green.
Melt the butter, add the flour and cook gently together for a few minutes without browning.
Whisk in the milk, stir in the mint and let the sauce bubble lightly for at least 5 minutes.
Then strain out the mint and stir in the cream and season to taste.
Stir in the prawns and serve as soon as they are heated through.