In a medium bowl, whisk together olive oil, soy sauce, lemon juice, Worcestershire sauce, balsamic vinegar, Dijon mustard, garlic, brown sugar, black pepper, salt, onion powder, and red pepper flakes.
Place the steak in a large resealable plastic bag or a shallow dish. Pour the marinade over the steak, seal or cover, and refrigerate for at least 2 hours or overnight for best results.
Preheat grill to high heat or broiler to high.
Remove steak from marinade and discard excess marinade. Pat steak dry for a better sear.
Grill or broil steak for 4-6 minutes per side, depending on desired doneness.
Let steak rest for 5-10 minutes before slicing against the grain. Garnish with chopped parsley if desired and serve.