1. Rinse and pat meat dry. Combine garlic, 3 tablespoons vinegar, and 1 tablespoon oil in a resealable plastic bag, seasoning with salt and pepper. Add beef and seal bag. Shake and rub bag to distribute marinade evenly. Let stand at room temperature 20 minutes.
2. Whisk together mustard and remaining 2 tablespoons vinegar in a small bowl. Salt and pepper to taste. Add remaining 5 tablespoons oil in a slow stream, whisking until thickened and emulsified. Combine lettuce, tomatoes, and capers in another bowl.
3. Preheat grill about 5 minutes. Cook steaks on a lightly oiled rack over high heat, turning once, until medium rare, about 5 minutes, depending on thickness. Transfer to a plate and let stand 10 minutes. Meanwhile, drizzle vinaigrette over salad, tossing to combine.
4. Cut steak into slices and serve topped with salad. Makes 8 servings.