Prep the sauce. In a bowl, whisk soy sauce, water, brown sugar (or honey), rice vinegar, and mirin if using. In a separate small cup, mix cornstarch with a tablespoon of water to make a slurry. Set both aside.

Cook the spaghetti. Boil in salted water until just shy of al dente. Reserve 1/2 cup pasta water, then drain. Toss with a teaspoon of oil to prevent sticking.

Season and sear the chicken. Pat the chicken dry. Season lightly with salt and pepper. Heat neutral oil in a large skillet over medium-high. Cook chicken in an even layer until browned and cooked through, about 5–7 minutes. Remove to a plate.

Sauté aromatics. In the same skillet, add a touch more oil if needed. Add garlic, ginger, and the white parts of the green onions. Cook 30–45 seconds until fragrant.

Add veggies (optional). Toss in bell pepper, snap peas, or broccoli. Stir-fry 2–3 minutes until crisp-tender.

Build the teriyaki. Pour in the soy mixture and bring to a gentle simmer. Stir the cornstarch slurry, then drizzle it in while stirring. Cook 1–2 minutes until the sauce thickens and turns glossy.

Combine. Return chicken to the pan, add cooked spaghetti, and toss to coat. If it seems tight, splash in reserved pasta water a little at a time until silky.

Finish. Turn off the heat. Stir in sesame oil, most of the toasted sesame seeds, and most of the green onion tops. Taste and adjust salt, sweetness, or acidity if needed.

Serve. Plate and sprinkle with remaining sesame seeds and green onions. Add a pinch of red pepper flakes if you like heat.