Put all marinade ingredients including flank steak into resealable, leak-proof bag. Seal and refrigerate overnight.
Preheat oven to 400°F.
Remove flank steak from marinade (discard marinade) and pat dry with paper towels.
Preheat large, heavy skillet (preferably cast-iron or stainless steel). Add 1 Tbsp of coconut oil. Once oil is hot, sear steak on both sides for 3 minutes per side.
Place skillet in oven. Roast for 12 minutes, or until meat thermometer inserted into thickest part reads 120°F. Let rest and cool to room temperature before slicing.
Cut flank steak in half lengthwise, then slice thinly against the grain.