In a medium bowl, whisk together the olive oil, soy sauce, Worcestershire sauce, and balsamic vinegar until well combined.
Add the brown sugar and stir until it is fully dissolved.
Stir in the garlic, onion powder, smoked paprika, black pepper, red pepper flakes (if using), dried oregano, and dried thyme. Mix until all the ingredients are well incorporated.
Place your beef (ribeye or sirloin, thinly sliced) into a large resealable plastic bag or a shallow dish.
Pour the marinade over the beef, ensuring all pieces are well coated.
Seal the bag or cover the dish with plastic wrap.
Refrigerate for at least 2 hours, or up to overnight for more intense flavour.