Whisk together olive oil, red wine vinegar, garlic, pepper, rosemary, oregano and salt.
Place the flat iron steak in a large size zip-top bag; pour marinade over and refrigerate for at least 30 minutes.
Preheat grill to medium-high heat and lightly oil the grate. Remove steak from marinade and allow to stand at room temperature for 15 minutes while the grill warms.
Mix together the butter, cheese, green onion and pepper and set aside.
Grill steak 6-8 minutes per side or to desired degree of doneness. Let rest 10 minutes before slicing thinly across the grain. Serve with a dollop of the gourmet butter.