Grilled Skirt Steak Salad with Arugula, Balsamic-Glazed Onions, Tomatoes, and Feta

In a large skillet over medium-high heat, heat 1 tablespoon olive oil. Add onion and cook until soft, 5 minutes. Add balsamic vinegar and brown sugar, and continue to cook for another 2 minutes, until very soft. Season with salt and pepper, then remove to bowl and let cool. Rub grill pan with 1 tablespoon olive oil and heat over high heat. Season skirt steak generously with salt and pepper, and grill steak for 3 minutes per side for medium rare. Remove steak to cutting board and let sit for 5 minutes. In large bowl, whisk together red wine vinegar with remaining 4 tablespoons of olive oil, then season with salt and pepper. Add arugula and tomatoes to vinaigrette and toss gently to combine. Thinly slice steak diagonally across the grain. Divide salad mixture among 4 plates. Top each with caramelized onions, steak, and crumbled feta, and serve immediately.