Preheat the oven to 200C/400F/Gas mark 6.
Heat the butter in a pan over a moderate heat. Add the watercress stalks and season with salt and freshly ground black pepper. Cook for 2-3 minutes, or until softened, then add the chicken stock and most of the watercress leaves, leaving a few leaves to garnish. Cook for a further minute, or until the watercress leaves have just wilted.
Blend the cooked watercress and the cream in a food processor until smooth. Strain the sauce through a fine sieve and return to the pan.
Add the carrot balls to the sauce and warm through.
Blanch the asparagus in a pan of salted boiling water for 1-2 minutes, drain and set aside.
Meanwhile, heat the olive oil in an ovenproof frying pan and fry the halibut fillets, skin-side down, for 2-3 minutes. Transfer the pan to the oven and roast the halibut for 3-4 minutes, or until cooked through.
To serve, place the halibut onto each of four serving plates. Spoon over the watercress sauce and arrange the asparagus around the plate. Garnish with the fresh chervil and reserved watercress leaves.
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