Finely mince garlic and rosemary to release flavor and help them blend smoothly into the marinade.
Whisk balsamic vinegar, soy sauce, and brown sugar in a bowl. Slowly add olive oil while whisking until slightly thick.
Stir in garlic, rosemary, salt, pepper, and red pepper flakes until well combined and fragrant.
Place steak in a zip-lock bag with the marinade. Seal and coat well. Refrigerate for 6 hours, flipping once.
Remove steak, pat dry completely, and discard the marinade.
Heat oil in a hot skillet. Sear steak for 3–5 minutes per side, depending on thickness and desired doneness.
Let steak rest 5 minutes, then slice thinly against the grain to keep it tender.