Preheat the oven to 180C/350F/Gas 4.
First make the chive cream sauce. Boil the cream in a saucepan for two minutes or until it has thickened slightly.
Remove from the heat and whisk in the mustards, horseradish and lemon juice and taste for seasoning, adding a little more salt or lemon juice as necessary. Keep the sauce warm but do not let it boil again or the flavours of the mustard and horseradish will be spoilt.
To prepare the salmon, check that it has been properly trimmed and is boneless. If there are any bones remaining, remove with a pair of tweezers. Cut the salmon into four equal portions. Season each piece with salt and pepper and then dredge lightly with flour.
For the horseradish crust, mix together the horseradish and egg yolk. Dip the topside of each piece of salmon into the horseradish/egg-yolk mixture, making sure that each fillet is generously coated.
Mix together the breadcrumbs and parsley and then dip each piece of fish into the breadcrumbs. Shape the crust by pressing this mixture onto the fillets gently but firmly, with your hands.
To cook the salmon, heat the oil in a heavy based frying pan until very hot. Add the fillets, crust side down, with the butter and cook for three minutes or until the breadcrumbs are beginning to crisp up nicely. Turn the fillets over and finish off the cooking in the oven for five minutes.
To serve, add the chives, to the sauce. Place each fillet on a warm plate, laying it on a bed of buttered, cooked vegetables if you wish (spinach is a great choice) and surround with a little of the sauce.