Carefully check your fillets to ensure there are no bones left. Then slice them diagonally into thin slices and distribute them over the 4 plates, cover, and place them cold in the refrigerator. Peel the grapefruit, making sure to cut off all the white pith as it is bitter, then cut the segments from between the membranes, also squeezing out the remaining juice. Cut the onion in half and then slice it as thinly as possible on the diagonal.
Put everything for the vinaigrette in a bowl, cut the chives into super fine rings, and stir them in. Taste and season with pepper and salt. Distribute the grapefruit segments and the onion over the sea bass, put it back in the refrigerator if you are not serving it immediately, as the citrus will start to cook the fish and it will become tougher. Just before serving, drizzle the whole thing with the vinaigrette. Optionally serve with toasted slices of bread.
Tips
If you serve the carpaccio as an appetizer, 125 to 150 g of fish per person is ideal. If you give him as a main dish, it goes to 150 to 180 g fish per person. Serve with extra toasted bread and possibly a salad.