01 - Bring a large pot of salted water to a boil and cook the spaghetti until al dente. Drain and reserve 1/2 cup of pasta water.
02 - Pat the salmon fillets dry and season both sides lightly with salt and pepper.
03 - Heat vegetable oil in a large skillet over medium-high heat. Place salmon skin-side down and cook for 3 to 4 minutes until the skin is crisp. Flip and continue cooking for 2 to 3 minutes until just cooked through. Remove fillets and set aside.
04 - Reduce skillet heat to medium. Add minced garlic and half the sliced scallions, sautéing for 1 minute until fragrant.
05 - Add soy sauce, mirin, sake (if using), and unsalted butter to the skillet. Stir until the butter melts and the sauce thickens slightly.
06 - Add drained spaghetti to the skillet. Toss thoroughly to coat with sauce, adding reserved pasta water as needed to achieve desired consistency. Drizzle with sesame oil.
07 - Flake salmon into large pieces and gently fold into the pasta, or arrange whole fillets on top.
08 - Divide pasta among plates. Sprinkle with remaining scallions and toasted sesame seeds. Serve with lemon wedges on the side if desired.