Combine the tuna, chopped onions, hot sauce, mayo, pepper, and sesame oil in a small bowl and mix to combine.
Place a medium to large skillet on low heat and add your English muffin halves all inside up onto the pan. Place even amounts of your tuna mixture to each English muffin half, top with one slice of tomato each, and sprinkle each evenly with cheese.
Cover and allow to cook until the cheese has melted (5-10 minutes).
Serve with French fries or a salad for a great, quick and tasty dinner treat.