Whisk together the olive oil, soy sauce, red wine vinegar, brown sugar, garlic, and black pepper in a 9x13" casserole dish. Place the steak into the dish flat, turning once. Cover tightly and refrigerate for at least two hours to overnight.
When ready to cook, place a 10 to 12 inch cast-iron skillet over medium high heat until hot. Carefully add the steak and cook for 3 to 4 minutes on each side for a medium rare doneness.
To cook steak to medium or medium well, preheat your oven to 350°. Add pan with steak inside and cook until the steak reaches the desired internal temperature to maintain juiciness. Ideal is at 130 F.
Bake time will be approximately 5 to 15 minutes longer. Remove once temp is what you want, then allow to rest for 10 minutes prior to slicing thinly against the grain.