PreparationMarinate steak: Whisk together Worcestershire sauce, lemon juice, oil, garlic, and sugar in a shallow dish. Add steak, turning to coat, and marinate at room temperature 15 minutes. Make dressing and prepare onion while steak marinates: Whisk together shallot, vinegar, mustard, 3 tablespoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a large bowl. Toss onion with remaining tablespoon oil and 1/4 teaspoon salt in a small bowl. Grill steak and onion: Prepare a gas grill for direct-heat cooking over medium-high heat. Pat steak dry and season with 1/4 teaspoon each of salt and pepper. Oil grill rack. Grill steak, covered, turning once, 8 to 10 minutes total for medium-rare. Transfer steak to a cutting board and let rest, uncovered, 5 minutes. While steak rests, grill onion in grill basket, covered, stirring occasionally, until charred and tender, 4 to 5 minutes. Thinly slice steak across the grain. Toss greens, onion, and steak with dressing and salt to taste. Cooks' note:Steak and onion can be grilled in a hot lightly oiled 2-burner grill pan over medium-high heat, 9 to 11 minutes.
PreparationMarinate steak: Whisk together Worcestershire sauce, lemon juice, oil, garlic, and sugar in a shallow dish. Add steak, turning to coat, and marinate at room temperature 15 minutes. Make dressing and prepare onion while steak marinates: Whisk together shallot, vinegar, mustard, 3 tablespoon oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a large bowl. Toss onion with remaining tablespoon oil and 1/4 teaspoon salt in a small bowl. Grill steak and onion: Prepare a gas grill for direct-heat cooking over medium-high heat. Pat steak dry and season with 1/4 teaspoon each of salt and pepper. Oil grill rack. Grill steak, covered, turning once, 8 to 10 minutes total for medium-rare. Transfer steak to a cutting board and let rest, uncovered, 5 minutes. While steak rests, grill onion in grill basket, covered, stirring occasionally, until charred and tender, 4 to 5 minutes. Thinly slice steak across the grain. Toss greens, onion, and steak with dressing and salt to taste. Cooks' note:Steak and onion can be grilled in a hot lightly oiled 2-burner grill pan over medium-high heat, 9 to 11 minutes.