Check your fillets carefully whether there are certainly no more bones. Then cut them diagonally into fine slices and divide over the 4 plates, cover and put cold in the fridge. Peel the grapefruit, certainly cut off all the white sheets because they are bitter, now cut the segments between the sheets, press the rest of the juice on it. Cut the onion in half and then diagonally in as thin as possible slices.
Put everything for the vinaigrette in a small bowl, cut the chives into super fine rings and stir. Taste and season with salt and pepper. Divide the grapefruit segments and the onion over the sea bass, put it back in a fridge if you do not immediately serve it, the citrus will cook the fish and then it becomes tougher. Just before serving, sprinkle the whole with the vinaigrette. Serve with toasted slices of bread if necessary.