Bring a large pot of salted water to a rolling boil. Add spaghetti and cook until al dente, 8 to 10 minutes. Reserve ½ cup of pasta water before draining.

Pat salmon fillets dry and season with salt and pepper on both sides.

In a skillet, heat vegetable oil until shimmering. Add salmon skin-side down and sear for 3 to 4 minutes until golden brown. Flip and cook for another 2 to 3 minutes, then remove from skillet.

Lower heat and add garlic and half of the scallions. Sauté for about 1 minute until fragrant.

Add soy sauce, mirin, and butter to the skillet. Stir until butter melts and sauce thickens, about 1-2 minutes.

Toss the drained spaghetti in the sauce, adding reserved pasta water to reach desired consistency. Drizzle sesame oil over pasta.

Flake the cooked salmon into large pieces and fold into the pasta, keeping some pieces whole for presentation.

Plate pasta and sprinkle with remaining scallions and toasted sesame seeds. Serve with lemon wedges if desired.