01 - Boil a large pot of salted water and cook spaghetti until al dente, about 10 minutes. Drain and reserve 1/2 cup pasta water.
02 - Pat salmon fillets dry with a paper towel and season lightly with salt and pepper.
03 - Heat vegetable oil in a large skillet over medium-high. Place fillets skin-side down, sear 3–4 minutes until skin crisps, then flip and cook another 2–3 minutes. Transfer to a plate and keep warm.
04 - Reduce skillet heat to medium, add minced garlic and half of the sliced scallions, sauté for 1 minute until fragrant.
05 - Stir in soy sauce, mirin, sake if using, and butter. Mix until butter melts and sauce slightly thickens.
06 - Add drained spaghetti to the skillet, toss to coat in sauce. Use reserved pasta water to adjust consistency. Drizzle with sesame oil.
07 - Flake salmon into large pieces and gently fold into pasta, or arrange whole fillets over the pasta.
08 - Distribute onto plates, finish with remaining scallions and toasted sesame seeds. Serve with lemon wedges if desired.