Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package directions until al dente. Before draining, reserve about 1/2 cup of the pasta water. Drain the pasta and set aside.

While the pasta is cooking, heat a large skillet over medium heat. Add 1 tablespoon of butter. Once melted, add the sliced mushrooms and sauté for 5-7 minutes until they are softened and lightly browned. Add the minced garlic and the white parts of the chopped green onions, and sauté for another 1-2 minutes until fragrant.

Add the drained tuna to the skillet with the mushrooms. Flake it gently with a spoon. Pour in the soy sauce, mirin, and sake (if using). Add the remaining 1 tablespoon of butter and black pepper. Stir gently to combine all ingredients and let the butter melt, creating a savory sauce.

Add the cooked and drained pasta to the skillet with the tuna and mushroom sauce. Pour in about 1/4 cup of the reserved pasta water. Toss everything together, allowing the pasta to absorb the flavors. If the sauce seems too dry, add a little more pasta water, one tablespoon at a time, until the desired consistency is reached.

Divide the pasta into serving bowls. Garnish generously with shredded nori seaweed, shichimi togarashi, and the green parts of the chopped green onions. Drizzle with a little toasted sesame oil if desired. Serve immediately.