Grill steaks in the usual manner and serve on a heated serving dish; garnish with sprigs of fresh watercress. Serve immediately with Bercy Sauce.
Poach diced beef marrow in boiling water; drain and cool. Simmer finely chopped shallots in white wine until the liquid is reduced to a third of the original quantity. Remove from heat and whisk until slightly cooled, then with pan over hot but not boiling water, gradually whisk in diced softened butter, stirring continuously until sauce is thickened. Stir in diced beef marrow, finely chopped parsley and lemon juice, and season to taste with salt and freshly ground black pepper.