Mix the seasoning base: In a medium bowl, whisk together Dijon mustard, minced garlic, and onion powder to build the flavor foundation.
Add the liquids: Pour in balsamic vinegar, soy sauce, Worcestershire sauce, and olive oil. Whisk again until smooth.
Prepare the steak: Place steak in a zip-top bag or shallow dish. Pour marinade over the meat, coating all sides evenly. Seal or cover.
Let it soak: Marinate in the fridge for at least 12 hours, ideally 24. Flip steak occasionally to keep coverage even.
Bring to room temperature: Remove steak from fridge 30 minutes before cooking for even heat distribution.
Heat your cooking surface: Brush grill grates or cast iron skillet with oil and heat until very hot and lightly smoking.
Cook the steak: For 2 cm thick steak, cook 2 minutes per side for medium rare. Use a thermometer to check doneness if needed.
Rest the steak: Transfer to a warm plate, loosely cover with foil, and rest for 5 minutes before slicing.
Serve and enjoy: Slice and serve as-is or with a dollop of garlic butter for extra flavor.