Combine sake cooking wine, sugar, sesame oil and cornstarch. Stir to dissolve cornstarch; set aside.
Cut pork into 1/2-inch slices; season with salt and pepper to taste. Press half the sesame seeds onto one side of each medallion. Melt butter in 12-inch saute pan. Cook pork, seed side down, over medium-high heat until lightly browned, about 8 minutes. While cooking, press remaining sesame seeds onto tops of medallions. Turn over and continue to cook 4-6 minutes. Increase heat to high, add sauce and cook 2 minutes or until sauce bubbles and thickens slightly. Serve pork medallions drizzled with sake sauce. Sprinkle with any remaining sesame seeds and green onion.