Preheat the oven to 220 ° C (7-8). Peel the quarters of grapefruit raw, removing the membrane that covers them and cut them into small dice by recovering the juice that flows. Lightly butter the inside of a sheet of parchment paper and place the fish, dices and grapefruit juice, the remaining butter and a pinch of salt. Close the pouch tightly and bake for 6 to 7 minutes. Serve warm decorated with coriander.