Bring a large pot of salted water to a boil. Cook spaghetti until al dente, about 8–9 minutes. Reserve 1/4 cup of pasta water and drain.

While pasta cooks, heat olive oil in a large skillet over medium heat. Add sliced garlic and sauté until golden but not burned, about 1–2 minutes.

Add crushed red chili flakes and cook for 30 seconds more to infuse the oil.

Add cooked spaghetti to the skillet and toss with the garlic chili oil. If needed, loosen with a splash of reserved pasta water.

Stir in grated Parmesan cheese until melted and well combined.

Plate and top with extra Parmesan and chopped parsley, if using. Serve immediately.