In a bowl, whisk together balsamic vinegar, soy sauce, olive oil, brown sugar, minced garlic, rosemary, kosher salt, ground black pepper, and red pepper flakes.
Pour the marinade into a zip-lock bag.
Add the steak, ensuring it’s well-coated.
Refrigerate for 6 hours to allow flavors to meld.
Remove the steak from the marinade, pat dry.
Heat olive oil in a skillet over medium-high heat.
Cook the steak to your desired doneness, ensuring a crispy exterior and flavorful interior.