1 1/2pounds assorted white fish fillets, carefully picked over for bones, rinsed, and patted dry
1cup snipped fresh dill leaves or minced chervil
2 egg whites
1 teaspoon fresh lemon juice
1 1/2cups heavy cream
4 tablespoons (1/2 stick) butter, cut into bits
Instructions
Combine the first 8 ingredients with water to cover in a pot and bring to a boil. Turn the heat to low and simmer for 30 minutes while you prepare the fish.
Combine the fish fillets and 1/2cup of the dill in a food processor and pulse until the fish is chopped and blended. Add the egg whites and process until just blended, no longer. Add the lemon juice, 1 cup of the cream, and a bit of salt and pepper and, again, process briefly, until the mixture is combined and smooth, but no longer.
When the stock is done, strain it into a clean pot and bring it to a simmer. Use 2 tablespoons or your hands to form 8 to 16 ovals of the fish mixture (see illustration below), depending on size. Place them in the pot and simmer for 5 minutes. Turn them and simmer for 5 minutes more. Remove the quenelles with a slotted spoon and drain on paper towels. Place on warmed plates and cover with foil.
Boil the stock until it is reduced to about 1 cup. Reduce the heat to low and stir in the butter a bit at a time. When the sauce thickens, add the remaining cream and 1/4cup of the remaining dill. Stir and heat through, but do not boil.
Serve the quenelles hot, with the sauce, garnished with the remaining dill.