Heat oil in a large skillet over high heat. Add flank steaks and cook well on both sides until browned and crisp. Reduce heat to medium and cook to desired doneness (10-12 minutes for medium, 8-10 minutes for med rare). Transfer to heated platter and keep warm.
Combine sake and water in same skillet and cook over medium high heat until reduced to 2 cups (10-15 minutes).
In a small cup combine soy sauce with water and then add to sake/water mixture along with the Kitchen Bouquet (or brown gravy sacue) and any juice from steak. Blend well and remove from heat but keep warm.
Slice rolls horontally and spread with butter and sprinkle with cheese. Toast in the broiler until golden brown.
Slice steak thinly on the diagonal, against the grain and then arrange steak slices on roll halves. Transfer to a large serving platter and serve with warm sauce.