Preheat oven to 425 F. Add potatoes to a parchment-lined baking sheet, and toss in the olive oil and onion salt. Bake for 20 minutes or until golden and crispy, flipping the potatoes halfway through.
While the potatoes are baking, marinate your onions. Add them to a small bowl along with the red wine vinegar and oregano and a pinch of salt. Massage with your hands to soften, and then set them aside.
Season your steaks with salt and pepper on both sides. To prepare your hanger steak, heat up a medium cast-iron skillet on medium-high heat. Once the pan is really hot, add the oil, and then add the steak. Cook for 3 to 5 minutes on each side depending on how thick your steak is and how you want it cooked. Look for an internal temperature of 125ºF for medium rare or 135ºF for medium. Remove steak from the pan, and let rest for 5-10 minutes before cutting into slices.
Prepare the dressing by adding ingredients to a small bowl or jar and whisking together with a fork until smooth.
Divide arugula into two bowls, and top with the crispy potatoes, tomatoes, and marinated red onions. Top with your sliced steak, and drizzle on the dressing. Finish off with some fresh parsley and some black and red pepper.
