Celery root, also known as celeriac, tastes like a cross between celery and parsley; choose a small, firm root. Martha made this recipe on Cooking School episode 407.
In a medium bowl, whisk together mayonnaise, creme fraiche, lemon juice, and mustard. Add cornichons, parsley, tarragon, capers, and salt; season with pepper. Whisk to combine. Fold celery root into dressing; serve atop lettuce leaves.