Cook leek in a smallsaucepan of boiling salted water untiltender, about 4 minutes.
Drain; transferto a colander set in a bowl of ice water.
Let cool; drain and transfer to paper towels.
Mix shallot, cornichons, mayonnaise,mustard, capers, dill, tarragon, and3 tablespoons water in a small bowl, addingmore water as needed to thin. Foldin leek; season with salt and pepper.
Combine garlic and 2 tablespoonsoil in a small bowl.
Heat remaining 1/2 cupoil in 2 large skillets over medium-highheat. Season mushrooms with salt andpepper. Cook each, cut side down, in askillet, pressing to flatten once they beginto soften, until golden and crisp, about3 minutes per side. Reduce heat tolow.
Drizzle garlic mixture around mushroomsand cook until garlic is golden,about 1 minute; turn mushrooms to coat.
Spoon leek rémoulade onto plates andtop with mushrooms.