Fill a saute pan fitted with a lid with water at least 1" deep.
Throw in parsley, lemongrass, tarragon and peppercorns. Then add the wine.
Cover the pan and allow the poaching liquid to come to a rolling boil then turn the heat down to low. Cook for 20 minutes to infuse the flavors.
Bring the liquid back up to a rolling boil. Turn heat back down to the lowest setting. This is done so that the fish will not come apart due to creating violent currents. When the boil settles down, using a thin, flat, slotted spatula, place the fillets gently in the water, and cover for 5 minutes. Remove the fillets to a platter and keep warm. If you need to poach in batches, remember to bring the liquid back to a rolling boil and then back down.
Heat lemon juice to boiling in a small saucepan.
One piece at a time, whisk butter into lemon juice, until the sauce is fully emulsified. You should have a thin and creamy sauce.
Whisk in chervil and your salt and pepper. Pour over fish and serve immediately.