Pour the milk into a large non-stick frying pan and add the water, lemon juice, onion, celery, carrot and peppercorns. Bring to the boil over moderate heat then cover and simmer for about 10 minutes.
Add the salmon steaks to the pan and bring the liquid back to the boil. Reduce the heat, cover and poach the fish for about 10 minutes or until the flesh flakes easily when tested with a fork.
Meanwhile, combine the horseradish, yoghurt, mayonnaise, chives and pepper to taste in a bowl. Stir until evenly mixed.
Using a large fish slice, transfer the salmon steaks to a platter lined with paper towels. Allow to drain. Discard the poaching liquid.
Transfer the salmon steaks to serving plates and spoon over the horseradish and chive sauce. Serve hot.