Ragout: Cut off and discard the green tops of the leeks. Split the bottom portion lengthwise, leaving the roots intact to keep the layers intact. Rinse thoroughly. Cut the leeks into three-inch lengths, discard the roots, then slice each section into eighth-inch thick matchsticks. Melt the three tablespoons of butter in a large saucepan over medium heat. Add the leeks and cook, stirring often, until they are softened and beginning to brown, about ten minutes. Add the wine and cook until most of the liquid evaporates. Add the cider and salt and gently boil uncoverd until the leeks cook down further and the liquid evaporates down below the level of the leeks, about ten minutes. Peel, core, and cut the apple into quarter-inch cubes and stir it into the leek mixture along with the lovage. Stir in the vinegar, taste, and season with pepper, and additional salt if necessary. (The ragout up to this point can be made as much as two days ahead.)

Fish: Preheat the oven to 425°. Trim any dark gray flesh from the skin side of the fish and check for and remove any stray bones. Cut the fillet into pieces appropriate for serving and sprinkle both sides with slat and pepper. Reheat the leek ragout on the stove until it simmers, then pour into an ungreased 12" x 9" shallow baking dish. Arrange the fish fillets on top of the hot ragout and pres them down so that their sides are submerged in the mixture but their tops are not covered by it. Cut a piece of parchment paper slightly larger than the baking dish and smear it with the softened butter. Press it, butter side down, directly on the fish. Bake the fish until it is milky, firm, and just barely translucent at the inside center, 10–15 minutes. It’s best to underbake the fish slightly because it will continue to cook once the dish is removed from the oven. Bring the dish to the table. Use a spatula to serve the fish fillets and then spoon some of the ragout and the sauce over each piece.

Herbal substitution: If you don’t have a friend with a lovage plant, substitute dill.