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

In large skillet, heat 1 tablespoon olive oil over medium-high heat. Add onion and sauté for 5 minutes, until softened. Add Balsamic vinegar and brown sugar, and continue to cook for another 2 minutes, until very soft. Season with salt and pepper. Remove to bowl and let cool. Rub grill pan with 1 tablespoon of olive oil and heat over high heat. Season skirt steak generously with salt and pepper. Grill steak for 3 minutes per side (6 minutes total) for medium rare. Remove steak to cutting board and let sit for 5 minutes. In large bowl, whisk together red wine vinegar with 4 tablespoons of olive oil. 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. Serve immediately.