Wash broccoli. Remove the leaves, peel the thick stem and cut into pieces. Cut the vegetables into small florets. Bring a little salted water to the boil in a half-height saucepan and cook the vegetables in it for about 12-15 minutes until al dente.
Put the crabs in a bowl, marinate with lemon juice and cover in the refrigerator.
Pour the cooked vegetables onto a colander and immediately immerse them in a bowl of ice-cold water. Take out again and drain well.
For the dressing, mix the crème fraiche and yoghurt together and season with lemon juice, salt, cayenne pepper and lemon zest to taste. Finally mix in the chives.
Arrange the vegetables on a platter, pour the dressing over them evenly and sprinkle with the shrimps. It is best to serve the salad lukewarm with a baguette.