Preheat the oven to 200°C/fan 180°C/gas mark 6. Clean and fillet each mackerel, lifting the bone out in one piece. Set the fillets aside.
Make the pickled rhubarb by scattering the rhubarb slices over a baking sheet in a single layer. Sprinkle over the sugar and drizzle on the white wine vinegar, then cover with foil. Place in the preheated oven to bake for about 12 minutes, until the rhubarb is just starting to soften but still keeping its shape. The exact time will depend on the size of the rhubarb pieces. Remove to a shallow dish and leave the rhubarb to cool in its marinade.
Next make the sauce. Put the rhubarb, sugar and ginger into a saucepan. Pour over 5 tablespoons of water and cook gently over a medium heat for about 10 minutes, or until the rhubarb has softened. Remove and purée in a blender or food processor.
Clarify the butter. Melt it slowly in a small pan. Carefully pour off the clear golden liquid and leave behind the milky part left at the bottom, which can be discarded. Beat the egg yolk and white wine vinegar in a bowl sitting over a pan of simmering water, making sure the water is not touching the bowl – this is your bain-marie. Then, whisking all the time, slowly pour the clarified butter into the egg yolk and vinegar. Keep whisking until you have a smooth, thick sauce. Check for seasoning, adding a little lemon juice and salt and pepper if needed. Gently fold in the rhubarb purée. Keep warm.
Brush the mackerel fillets with a little oil. Place on a griddle pan over a medium heat, skin side down, for 2 minutes, then turn over and cook for another 2 minutes or until just cooked.
Next take the watercress and dip it into the liquid of the pickled rhubarb. Serve alongside the griddled mackerel fillets, rhubarb hollandaise and pickled rhubarb.