Season Alaska salmon steaks/fillets lightly with salt and pepper. Place in a pan just large enough to hold the salmon in 1 layer. Add the champagne, lime juice and just enough water to cover the fish. Remove the fish and bring the liquid to a boil. Return the salmon steaks/fillets to the pan. Top each with an onion slice, capers and tarragon sprig. Reduce heat to a simmer, cover pan with foil and poach at no more than a simmer for 6 to 10 minutes (depending on the thickness of the salmon). Remove salmon steaks/fillets from the liquid and place on 4 warm serving plates. Top each piece of fish with 1 ounce of the mustard mixture and serve.