In a medium bowl or zip-top bag, combine soy sauce, olive oil, balsamic vinegar, Worcestershire sauce, garlic, rosemary, pepper, mustard, brown sugar, and red pepper flakes. Whisk or shake to combine
Add steak to the marinade, ensuring it’s fully coated. Seal and refrigerate. Marinate for at least 1 hour, ideally 4–8 hours
Remove steak from marinade and let it come to room temperature. Pat dry with paper towels for better searing
Cook to desired temperature on grill or skillet. Rest steak for 5 minutes before slicing. Serve and enjoy!