Orange, lemon and dill grilled salmon

Grated zest and juice of 1 orange and 1 lemon
1 tablespoon olive oil
1 teaspoon whole grain mustard
Salt and freshly ground black pepper
4 salmon steaks
40 g (1 1/2 oz) butter
2 tablespoons freshly chopped dill
fresh watercress to serve, optional

Blend together the orange and lemon zest and juice, oil, mustard and seasoning. Place the salmon steaks in a dish, pour over the marinade and leave for 1-2 hours. Meanwhile, blend together the butter, dill and seasoning. Place on a sheet of food wrap and roll up to form a "sausage" shape. Chill until firm then cut into 8 rounds. Grill the salmon under a preheated moderate grill for 15-20 minutes, turning and basting occasionally with the marinade. Serve immediately on a bed of watercress with the herb butter rounds.