Blanch the fine beans in boiling water for 3 mins. Drain and rinse in cold water.
In a large bowl, toss the beans with the watercress, cannellini beans, beetroot and red onion, then pile everything onto a large serving platter.
Mix all the dressing ingredients together in a small jug, season to taste, then drizzle over the salad to serve.
Cook's tip: You could make this salad more substantial by adding some flaked smoked mackerel. The flavours of the fish and the horseradish work well together.