Add steak to a large Ziploc bag or baking dish and toss in balsamic vinegar, garlic and brown sugar. Let marinate 20 minutes at room temperature.
Heat a grill or grill pan to high. Rub steak with vegetable oil and season generously with salt and pepper.
Grill until desired doneness, 3 minutes per side for medium rare. Let rest 5 minutes, then slice.
Meanwhile, make dressing: In a small bowl whisk together olive oil and lemon juice and season with salt and pepper.
Arrange salad: In a large serving bowl, add arugula, peaches, blue cheese or feta, and steak. Drizzle with dressing and gently toss. Serve immediately.