Top and tail the grapefruit, then slice off the peel and pith. Cut through the membranes to release fine segments. Catch these in a bowl, along with any juice. Stir the segments into the hollandaise along with a few tbsp of juice and the basil. Set aside.
Boil 1 litre water in a wide shallow pan with the herbs, onion, vinegar and seasoning for 5 mins. Put in the salmon, pour over extra water to cover and poach gently until just firm, about 7 mins. Remove and drain, then place on plates. Spoon over the sauce and serve with a green salad and potatoes.