Preheat oven to 400 degrees. Mince garlic and parsley and set aside so they are ready to add to sauce.
Divide flank steak into 4 equal portions, pat dry and season with kosher salt and freshly ground black pepper.
In a large cast iron pan, heat oil over medium high heat until pan is very hot. Add 1 tablespoon of butter to pan and let melt. Sear steaks on both sides, about 3 minutes per side.
Place cast iron skillet into oven and cook for another 3 minutes, or until steaks reach 125 degrees on an instant read thermometer.
Carefully remove cast iron skillet from oven and return to stovetop, handle will be very hot. Remove steaks from pan and set aside to rest, then prepare pan sauce.
Heat skillet over medium low heat, add 1 tablespoon of butter and garlic to skillet and saute for 1-2 minutes.
Add white wine and any juices from resting steak to pan and increase heat to medium high. Bring to boil and let reduce about 5 minutes until slightly thickened, scraping up brown bits from pan as it reduces.
Remove pan from heat, add final tablespoon of butter and freshly chopped parsley to sauce and season with additional slat and pepper as needed.
Slice flank steak portions against the grain, spoon sauce over slices and serve.