Shrimp and Hearts of Palm Rémoulade

INGREDIENTS

• 1/2 teaspoon Worcestershire sauce
• 1/2 teaspoon paprika
• 1 cup mayonnaise
• 1 onion, quartered
• Lemon wedges
• 1 tablespoon fresh lemon juice
• 1 garlic clove, peeled
• 2 celery stalks, 1 coarsely chopped, 1 thinly sliced on the diagonal
• 1 celery stalk, coarsely chopped
• Butter lettuce leaves
• 4 sprigs thyme
• 1 teaspoon whole black peppercorns plus freshly ground black pepper
• 1 14-ounce can hearts of palm, drained, sliced on the diagonal
• 2 bay leaves
• 1 tablespoon prepared horseradish
• 1 small shallot, coarsely chopped
• 1 teaspoon yellow mustard seeds
• 1 tablespoon kosher salt plus more
• 2 pounds uncooked unpeeled large shrimp
• 1/2 cup loosely packed flat-leaf parsley leaves
• 2 tablespoons whole grain mustard