For the marinade: combine all ingredients and mix well. Set aside.
Butterfly steak. With one of the long sides not short sides, facing you, run knife through middle thickness leaving ½-inch at the long opposite end as a “hinge.” Do not cut all the way through.
Place steak in 9 x 13 pan, exposing all cut sides, and marinate overnight in refrigerator.
(Next day) Heat grill to medium heat.
Lay steak open. Place mushrooms on steak to cover completely. Sprinkle with 1 tablespoon parsley.
Cover with basil leaves and sprinkle with Parmesan along with the remaining 2 tablespoons parsley and pepper.
Roll up from a long side jellyroll style. Tie at 2-3 inch intervals with kitchen string.
Place steak on grill, cooking until desired doneness or until internal temperature reaches 145°F, turning frequently.
Transfer steak to a cutting board, tent with foil, and let rest for 10 minutes.
Remove twine and slice steak, crosswise, into ½-inch thick slices.
OVEN BRAISED SIRLOIN STEAK: Preheat oven to 350°F. Prepare steak per directions through step 7. Combine 1/4 cup olive oil, balsamic vinegar, garlic cloves, parsley and black pepper in shallow baking dish. Bake 15 minutes; baste with juices and bake an additional 20 minutes. Let meat rest at least 10 minutes before slicing.