In a large pot, bring water to a boil and cook the spaghetti or linguine according to the package instructions until al dente. Drain and set aside.

In a separate pan or wok, heat the vegetable oil over medium heat. Add the minced garlic and chopped onion, sautéing until they turn fragrant and slightly golden. Next, add the chicken breast and cook until it's no longer pink in the center.

Once the chicken is cooked, add the shiitake mushrooms, red and green bell peppers to the pan. Stir-fry for a few minutes until the vegetables are slightly tender. Now, it's time to introduce the flavorful sauces. Pour in the oyster sauce, soy sauce, hoisin sauce, and chili garlic sauce, combining them well with the ingredients in the pan.

To achieve the desired consistency, add the chicken stock to the pan and bring it to a simmer. Gradually pour in the cornstarch mixture while stirring continuously. This will help thicken the sauce and give it a luscious texture. Season with salt and pepper to taste.

Now that the sauce is ready, it's time to bring all the elements together. Add the cooked pasta to the pan and toss it gently to coat it evenly with the savory sauce. Sprinkle the chopped green onions on top for a burst of freshness and garnish with crushed peanuts for that extra crunch.