- 2 1 1/2 pounds flank steaks
- Kosher salt
- 2 teaspoons (packed) light brown sugar
- 1 teaspoon paprika
- 1/4 teaspoon cayenne pepper
- 1/4 teaspoon freshly ground black pepper
- Vegetable oil (for brushing)
- 1 cup finely chopped red onion
- 3 tablespoons Sherry vinegar, divided
- 2 pound cherry or grape tomatoes, halved
- 1 cup chopped celery hearts (inner stalks and leaves; from 1 bunch)
- 1/2 cup chopped brined green olives plus 2 tablespoons olive brine
- 2 tablespoons prepared horseradish
- 1 tablespoon Worcestershire sauce
- 1 teaspoon hot pepper sauce
- 1/2 teaspoon celery seeds
- 1/4 cup extra-virgin olive oil
- Kosher salt and freshly ground black pepper
- Place steaks on a large rimmed baking sheet; season with salt. Mix sugar and next 3 ingredients in a small bowl; rub all over steaks. Cover; refrigerate 1-3 hours.
- Let steaks stand at room temperature for 1 hour. Build a medium-hot fire in a charcoal grill, or heat a gas grill to high. Brush grates with oil. Grill to desired doneness, about 5-7 minutes per side for medium-rare. Let rest for 10 minutes. DO AHEAD: Can be made 1 day ahead. Let cool; cover and chill. Bring to room temperature before slicing.
- Mix onion and 1 tablespoon vinegar in a large bowl. Let macerate 10 minutes, tossing often. Add tomatoes, celery, and olives.
- Whisk remaining 2 tablespoon vinegar, olive brine, horseradish, and next 3 ingredients in a medium bowl. Slowly whisk in oil. Add to bowl with tomato mixture; toss to coat. Season with salt and pepper. DO AHEAD: Can be made 4 hours ahead. Cover; chill.
- Cut steak crosswise into 1/4″-thick slices. Serve salad with steak.