Boil the Spaghetti: Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente, about 8-10 minutes. Reserve 1/2 cup pasta water, then drain.

Prepare the Mushrooms: Rinse and dry mushrooms, slice them into bite-sized pieces.

Sauté the Garlic: Heat olive oil in a skillet, add minced garlic and chili flakes, sauté until fragrant, about 1 minute.

Cook the Mushrooms: Add sliced mushrooms, season with salt and pepper, sauté for about 7 minutes until browned.

Combine Spaghetti and Sauce: Add cooked spaghetti to skillet, toss with mushrooms, add parsley and Parmesan cheese if using.

Adjust the Sauce: Gradually mix in reserved pasta water until sauce is smooth and coats the spaghetti.

Serve and Garnish: Serve immediately in warm bowls, garnish with extra Parmesan and chili flakes.