Whisk together the sake, miso, mirin, soy sauce, ginger, garlic, cayenne and brown sugar in a bowl. Add the Mahi Mahi to a ziplock flipping over in the marinade to coat. Cover and refrigerate for 2 hours. Bring Mahi Mahi to room temperature, about 15 minutes, then remove the fish from the marinade and shake off excess. Save the remaining marinade. Heat a small non-stick pan over medium high heat with a small drizzle of sesame oil. Add the fish and cook until done, about 3-4 minutes per side. Remove fish from pan to a serving plate. Pour the reserved marinade in the pan and cook until it is reduced by half. Add the scallions and sesame seeds, tossing to mix. Pour over the Mahi Mahi. Squeeze the lime on top, and serve.