Cook spaghetti a minute or 2 shorter than cooking time (it should still have a little white in the center when you bite it).
Heat pan over medium heat and add cooking oil.
When the oil get hot, quickly saute garlic, onion then add the chicken fillet, season lightly with salt and ground black pepper; cook for a few minutes
Next, add red pepper flakes, shiitake mushroom and peanuts. Take out of the pan and set aside.
Combine oyster sauce, char siu sauce, mirin and sesame oil in a small bowl; mix well and add to the pan, bring to boil then lower heat and let simmer for a few minutes
Add cooked spaghetti to the pan, mix well to coat the pasta evenly then add the cooked chicken mixture. Mix well.
Finish by adding green onions or parsley and a few more drops of sesame oil. Serve.