Flank Steak with Bloody Mary Tomato Salad

INGREDIENTS

• 1 cup chopped celery hearts (inner stalks and leaves; from 1 bunch)
• 2 pound cherry or grape tomatoes, halved
• 2 tablespoons prepared horseradish
• Kosher salt and freshly ground black pepper
• 1/4 teaspoon freshly ground black pepper
• 1/2 cup chopped brined green olives plus 2 tablespoons olive brine
• 1 teaspoon paprika
• 1 cup finely chopped red onion
• 3 tablespoons Sherry vinegar, divided
• 2 teaspoons (packed) light brown sugar
• 1/4 teaspoon cayenne pepper
• 2 1 1/2 pounds flank steaks
• 1/2 teaspoon celery seeds
• 1 teaspoon hot pepper sauce
• 1/4 cup extra-virgin olive oil
• 1 tablespoon Worcestershire sauce
• Kosher salt
• Vegetable oil (for brushing)