Boil the noodles according to the package instructions until tender. Drain and set aside.
In a small bowl, mix soy sauce, teriyaki sauce, brown sugar, and mirin to make the sauce.
In a large skillet or wok, heat butter and sesame oil over medium heat.
Add chopped onions, minced garlic, and ginger. Sauté for 2–3 minutes until fragrant.
Toss in the cooked noodles and pour the sauce over them.
Stir-fry everything for 3–5 minutes until the noodles are well-coated and heated through.
Season with salt and pepper as needed.
Sprinkle sesame seeds on top before serving.