For the pâté, remove the flesh from the fish, discarding all the skin and bones, and place into a blender. Add the lemon juice and blend quickly with the cream to a slightly coarse pâté. Transfer the pâté from the blender to a bowl, add the chives and season to taste with sea salt and freshly ground black pepper. Spoon the pâté into a pot, smooth over the surface and place in the fridge.
To make the dressing, place both the vinegars in a bowl. Whisk in 225ml/8fl oz of the olive oil then check the flavour for balance of oil and acidity, adding more oil if needed.
Shape the pâté into quenelles. Dress the salad leaves with a little of the vinaigrette and cut the eggs in half. Place the beetroot in overlapping circles on a plate, place the salad leaves on top of the beetroot, top with the pâté and the egg and garnish with a few chervil leaves. Serve with slices of warm Melba toast.