Over high heat, reduce balsamic vinegar by about 60 percent. Syrup will thicken as it cools. This step should be done well ahead; reduced balsamic vinegar can be stored at room temperature indefinitely.
To prepare dressing, add 1/2 cup olive oil, garlic and capers to a small sauté pan. Over medium heat, cook until garlic is lightly browned. Whisk in honey mustard. Then, stir in the rosemary and pepper to taste; keep warm.
Rub steaks with the remaining olive oil and season liberally with salt and pepper. Grill steak to desired doneness. Transfer to a cutting board and let rest 5 minutes.
Arrange arugula on a serving platter or plates. Slice the steak across the grain into thin slices and arrange on top of arugula. Pour warm dressing over top, which will begin to wilt the arugula. Drizzle with reduced balsamic vinegar. Serve immediately with lemon wedges.