How to Make It
Rinse and dry the fish. Set the fillet in a deep 9-by-13-inch baking dish. In a large bowl, whisk the water with the salt until the salt is dissolved, then add the dill seeds, anise seeds, coriander seeds, herb sprigs, garlic and shallot. Pour the brine over the fish and evenly distribute the aromatics. Top the fillet with a sheet of paper towel, pressing gently to moisten the sheet. Wrap the entire dish in plastic wrap. Refrigerate for 3 days, turning the fish in its brine, topping it with a fresh paper towel and rewrapping in plastic wrap after 36 hours.
Drain and discard the brine and scrape the aromatics off the fish fillet. Place the fillet skin side down on a rack set over a baking sheet and refrigerate uncovered until the surface is mostly dry but slightly tacky, about 2 hours.
In a bowl, combine the lemon juice with the garlic and shallots and let stand for 20 minutes. Stir in the parsley, mint, dill, chervil, lemon zest, anise seeds, Aleppo pepper and oil and season with salt and pepper.
Place the fish on a work surface. Using a very sharp, thin-bladed knife, cut the fish crosswise into very thin slices, running the blade horizontally once it reaches the skin to free each slice. Transfer the slices to a platter and serve with the herb sauce, crème fraîche, cornichons, dark bread and mustard.