Divide the pastry in two and roll each piece out as thin as possible. Finish by stretching them over the backs of your hands (the pastry should now be so thin you can see through it) to give 2 sheets about 60 X 45 cm/24 X 18 inches. Allow the pastry to dry out a little, then cut each sheet into 12 squares of about 15 cm/6 inches. Take 6 round, high-sided tins, such as brioche tins, about 10 cm/4 inches across and about 5 cm/2 inches deep. Lay 4 pastry squares in each tin, one on top of the other so that the points of the squares form a petal formation. Bake in the oven for about 10 minutes or until golden brown.
Bring the bouillon to the boil, then plunge in the crab and simmer for 12 minutes. Remove the pan from the heat and allow the crab to go cold in the stock. When cold, remove all of the meat, both from the body and from the legs and claws. Put the white meat to one side, and place the dark meat in a food processor or blender and process until fine.
Skin and segment 2 of the grapefruits, and squeeze the juice from the third. Reserve 12 nice segments for garnish, cut the rest into 3 or 4 pieces and mix with the white crab meat. Bring the grapefruit juice and fish stock to the boil and reduce by three-quarters. Add the cream and the brown crab meat to the stock, return to the boil and reduce again by almost a half. Allow the sauce to go cold.