For marinade, mix bourbon, soy sauce, brown sugar, lemon juice, Worcestershire, garlic powder and rosemary.
Place steak in a Ziploc bag or a non-reactive baking dish. Pour marinade over steak and allow steak to marinate overnight.
When ready to cook, preheat oven to 450.
Place mushrooms and sage on a baking sheet.
Pour 3 tablespoons of olive oil on mushrooms and mix well with your hands.
Roast for 20 minutes.
Meanwhile, butterfly flank steak.
Spread cheese and mushrooms over butterflied flank steak, leaving one inch from edges.
Roll flank steak and secured with kitchen twine.
Tuck and secure ends of flank steak with toothpicks so filling does not escape.
Heat oven safe grill pan (or regular oven safe frying pan) over medium high heat. A grill cast iron is great.
Add 1 tablespoon of grape seed oil. Sear flank steak for 2-3 minutes on each side (turning four times).
Move pan to oven and allow flank steak to roast for 20-25 minutes (for medium).
Allow steak to rest 5 minutes before slicing and serving.