Fillet of halibut with watercress sauce and seasonal vegetables

Ingredients 110g/4oz butter 400g/14¼oz watercress, stalks finely chopped, leaves reserved 200ml/7fl oz chicken stock 100ml/3½fl oz double cream 2 carrots, scooped into balls using a small melon baller 1 bunch asparagus, trimmed 2 tbsp olive …