Place salmon in a shallow pan large enough to hold in a single layer.
Add champagne, lemon juice, zest and enough water to just cover fillets.
Remove fish and bring poaching liquid to a boil.
Lay fish skin sides down back in pan and top with onion slices, capers, olives, tarragon, salt and pepper. Reduce heat until liquid is at a very low simmer and poach 4 to 6 minutes, depending on thickness of the fish.
Fish is done when flesh is opaque and flakes easily with a fork.
Remove fish from liquid, along with onions, capers, olives, and tarragon.