In a large bowl, whisk together soy sauce, balsamic vinegar, Worcestershire sauce, brown sugar, finely diced onion, minced garlic, and parsley.
Into the same bowl add the steaks to the marinade and cover with plastic wrap. You could also add the steaks to a resealable bag and pour the marinade into the bag.
Marinate for 1 hour up to overnight.
Notes
Bring steaks to room temperature before grilling for the best results.
Pat steaks dry to remove excess marinade. If the steaks are not dry it is hard to get a good sear.
Be sure to salt your steak well before grilling.
Preheat the grill to 500 degrees. It's always best to start with a very hot grill.
If cooking in a cast iron skillet be sure to start with a hot skillet.
Oil the grill grates, not the steaks using tongs and a paper towel.
Get a perfectly cooked steak by using a meat thermometer.
Be sure to let steak rest for 5 - 10 minutes after removing them from the grill. This will allow all of the juices to redistribute making every bite delicious.