Bring a large pot of salted water to a boil. Cook the spaghetti or noodles according to package instructions until al dente. Drain and set aside.
Heat vegetable oil in a large skillet or wok over medium heat. Add minced garlic and chopped onion, stirring until fragrant and lightly golden.
Toss in the diced carrots, green peas, and mushrooms. Cook for 3–4 minutes, stirring occasionally, until the vegetables are slightly tender.
In a small bowl, combine soy sauce, oyster sauce, hoisin sauce, sugar, and water or broth. Mix well and pour into the skillet with the vegetables.
Stir the dissolved cornstarch mixture into the skillet. Allow the sauce to simmer for 1–2 minutes until it thickens and coats the vegetables.
Toss the cooked noodles into the skillet. Mix thoroughly to ensure the noodles are well-coated with the sauce.
Transfer the noodles to serving plates. Garnish with sesame seeds if desired. Serve hot and enjoy!