"Create a fusion Shepherd’s Pie recipe combining traditional British style with Japanese flavors. The filling should be Japanese-style tuna mayo made with tuna, Japanese mayonnaise, and light seasoning. Include complementary Japanese ingredients like soy sauce, mirin, sesame oil, or nori as appropriate. The top layer should be mashed potatoes with subtle Japanese elements like miso, furikake, or butter-soy seasoning. The dish should be oven-baked like a classic shepherd’s pie. Provide exact ingredient quantities, step-by-step cooking instructions, preparation and cooking time, and tips for balancing the Japanese and Western flavors. The recipe should be creative but practical for home cooking."