In a large resealable plastic bag or baking dish, mix together the steak, vinegar, garlic, and brown sugar. Let it marinate for 20 minutes at room temperature.
Heat up your grill or grill pan over high heat. Take the steak out of the marinade, coat it with vegetable oil, and season generously with salt and pepper.
Grill to your preferred doneness, about 3 minutes per side for medium-rare. Allow it to rest for 5 to 10 minutes, then slice against the grain.
Whip up the dressing: In a small bowl, whisk together the olive oil and lemon juice, then season with salt and pepper.
Assemble the salad: In a large serving bowl, toss together the arugula, peaches, blue cheese or feta, and steak slices. Drizzle with the dressing and give it a gentle toss. Enjoy your flavor-packed masterpiece!