01 - Bring a large pot of salted water to a boil. Cook the spaghetti until al dente according to package instructions. Drain, reserving 1/2 cup of the pasta cooking water.
02 - Pat salmon fillets dry and season lightly with salt and pepper.
03 - Heat vegetable oil in a large skillet over medium-high heat. Place salmon fillets skin-side down and cook for 3–4 minutes until the skin is crisp. Flip and continue cooking for 2–3 minutes until just cooked through. Remove salmon and set aside.
04 - Lower the heat to medium. In the same skillet, add minced garlic and half of the sliced scallions. Sauté for about 1 minute until aromatic.
05 - Pour soy sauce, mirin, optional sake, and butter into the skillet. Stir until the butter has melted and the sauce becomes slightly thickened.
06 - Add the cooked spaghetti to the skillet and toss to coat in the sauce. Use reserved pasta water as needed to adjust the sauce consistency. Drizzle the sesame oil over the pasta.
07 - Flake the salmon into generous pieces and fold gently into the pasta, or serve whole fillets atop the pasta.
08 - Divide pasta among plates. Top with remaining sliced scallions and toasted sesame seeds. Serve with lemon wedges if preferred.