Heat the butter, sugar, star anise and 1 tablespoon of cold water in a medium pan. Add the rhubarb, cook for 2-3 minutes, until softening, then add the sherry vinegar and cook for a further 1 minute. Strain through a sieve, reserving the liquid. Return the liquid to the pan and reduce for 2 minutes, until thick and syrupy. Mix into the cooked rhubarb and set aside.
Heat 1 tablespoon of the oil in a frying pan over a medium heat, and cook the mackerel fillets skin side down for 3-4 minutes, until crisp. Gently turn and cook for 1 minute.
In a bowl, combine the horseradish and crème fraîche. In a second bowl, whisk together the white wine vinegar and remaining olive oil, then drizzle over the watercress.
Divide the toasts between plates and top with the dressed watercress and mackerel fillets. Serve with the rhubarb and horseradish crème fraîche on the side.