Marinate: In a mixing bowl, combine soy sauce, olive oil, balsamic vinegar, Worcestershire sauce, minced garlic, Dijon mustard, brown sugar, black pepper, salt, and red pepper flakes. Whisk until well mixed.
Prepare Steak Tips: Place the beef tips in a marinating dish or resealable plastic bag. Pour the marinade over the steak tips, making sure they are well-coated. Seal and refrigerate for at least 2 hours, or overnight for best results.
Preheat Grill: Heat your grill to medium-high heat. Remove the steak tips from the marinade, letting the excess drip off.
Grill: Place the steak tips on the grill. Cook for 4-5 minutes on each side, or until they reach the desired doneness.
Rest: Remove the steak tips from the grill and let them rest for a few minutes under aluminum foil to retain juiciness.
Serve: Garnish with fresh parsley and serve hot.