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Put the vinegar, peppercorns and coriander seeds (if using) into a small pan with 1 tbsp water. Simmer until reduced to 1 tsp, then remove from the heat. If it reduces too much, add a splash of water.
Segment the grapefruit and put the segments onto kitchen paper. Squeeze the juice from the middle of the fruit.
Melt the butter in a small pan and keep it warm. Put the egg yolks into a blender or food processor with a generous pinch of salt. Strain the reduced vinegar on top, then discard the pepper and coriander seeds. Add 1 tsp grapefruit juice, then whizz for a few seconds.
With the motor on the blender running, pour in about 1 tbsp butter and the cream. Gradually add the rest of the butter, allowing it to fully emulsify after each addition to make a thick, smooth sauce. Keep the butter warm on the hob in-between pours. Add 1 tsp more grapefruit juice to loosen, then season. Stir in the segmented grapefruit and break it up a little. Keep the sauce warm while you cook the fish.
Slash, then season the sea bass. Heat a non-stick frying pan over a medium-high heat. Heat the oil, then fry a fillet or two, skin-side down, for 4-5 mins, or until the flesh has changed colour almost all the way through and the skin is crisp. Flip the fish over, cook for 30 secs, then remove to a plate and keep warm while you fry the remaining fillets. Serve with the hollandaise and steamed greens.