Combine the olive oil, balsamic vinegar, Worcestershire sauce, and soy sauce in a medium bowl and whisk until smooth. Add the Dijon mustard, minced garlic, brown sugar, and red pepper flakes, then continue stirring until the sugar is fully dissolved and everything is evenly incorporated. Taste the marinade and adjust if needed — a small extra splash of Worcestershire can deepen the flavor.

Use paper towels to pat the steaks completely dry on both sides, then season each side evenly with the kosher salt and freshly cracked black pepper. Arrange the seasoned steaks in a shallow container or resealable bag, pour the marinade over them, and turn to coat all sides thoroughly. Seal and refrigerate for at least 4 hours, or overnight for the deepest flavor.

Pull the steaks from the refrigerator 15 to 20 minutes before you plan to cook them so they can come closer to room temperature, which promotes more even cooking. Heat a grill or cast-iron skillet over medium-high heat until very hot and just starting to smoke. Lift the steaks out of the marinade and let the excess drip off, then place them on the hot cooking surface. Cook for 5 to 7 minutes per side for medium-rare, depending on thickness, keeping them undisturbed so a deep crust forms. Transfer to a cutting board and allow the steaks to rest for 5 minutes before serving.