1. Preheat a grill to high heat. Mix the brown sugar, 2 tablespoons salt, and 1 tablespoon pepper together in a small bowl. Rub into the steaks and set aside for 15 to 20 minutes.
2. Drizzle the onion with olive oil and season with some salt. Grill on each side until soft and darkly charred, about 8 minutes. Set aside.
3. Grill the steaks on high heat until crisp, about 2 1/2 minutes per side. Set aside to rest for 5 minutes, and then slice the steaks across the grain. Serve over the grilled onions with a lime wedge.
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