Prepare the marinade by mixing soy sauce, olive oil, red wine vinegar, minced garlic, brown sugar, black pepper, and salt.
Place the flank steak in a resealable bag or shallow dish and pour the marinade over it.
Seal the bag or cover the dish and refrigerate for at least 1 hour or overnight.
Preheat your grill or skillet over medium-high heat.
Remove the flank steak from the marinade and allow excess to drip off.
Cook the steak on the grill or skillet for 5-7 minutes on each side for medium-rare.
Check with a meat thermometer; aim for 130-135°F for medium-rare.
Remove from heat and let it rest for 5-10 minutes.
Slice the steak against the grain into thin strips.
Serve immediately, garnished with fresh herbs if desired.