Step 1

Cook leek in a small saucepan of boiling salted water until tender, about 4 minutes. Drain; transfer to a colander set in a bowl of ice water. Let cool; drain and transfer to paper towels.

Step 2

Mix shallot, cornichons, mayonnaise, mustard, capers, dill, tarragon, and 3 tablespoons water in a small bowl, adding more water as needed to thin. Fold in leek; season with salt and pepper.

Step 3

Combine garlic and 2 tablespoons oil in a small bowl. Heat remaining 1/2 cup oil in 2 large skillets over medium-high heat. Season mushrooms with salt and pepper. Cook each, cut side down, in a skillet, pressing to flatten once they begin to soften, until golden and crisp, about 3 minutes per side. Reduce heat to low. Drizzle garlic mixture around mushrooms and cook until garlic is golden, about 1 minute; turn mushrooms to coat.

Step 4

Spoon leek rémoulade onto plates and top with mushrooms.