Seared Maitake Mushrooms

Name: Seared Maitake Mushrooms
Ingredients: 1 leek, white and pale-green parts only, thinly sliced; Kosher salt; ½ small shallot, finely chopped; 2 cornichons, finely chopped; ⅓ cup vegan or regular mayonnaise; 2 Tbsp. Dijon mustard; 1 Tbsp. capers, finely chopped; 1 Tbsp. chopped fresh dill; 1 Tbsp. chopped fresh tarragon; Freshly ground black pepper; 2 garlic cloves, finely chopped; 2 Tbsp. plus ½ cup olive oil; 2 8-oz. maitake mushrooms, cleaned, halved through the stem; Kosher salt and freshly ground black pepper