First, bring a large pot of generously salted water to a rolling boil. Add your spaghetti and cook according to the package directions until it reaches al dente. This means it should still have a slight bite. Just before draining, reserve about one cup of the starchy pasta water.
While the pasta is cooking, heat the olive oil in a large skillet or pan over medium heat. Add the thinly sliced garlic and sauté gently. You want the garlic to turn a light golden brown and become fragrant, not burnt, as burnt garlic will make the entire dish bitter.
Once the garlic is golden, stir in the red chili flakes. Let them sizzle in the oil for about 30 seconds to a minute. This step blooms the spice, releasing its flavor and a gentle heat into the oil, creating the signature taste of the dish. Be careful not to let them burn.
Drain the cooked spaghetti and immediately add it to the skillet with the garlic and chili oil. Toss everything together thoroughly to ensure every strand of pasta is coated in the flavorful oil. If the pasta looks a bit dry, add a splash of the reserved pasta water to create a light, silky sauce.
Remove the skillet from the heat and stir in the finely chopped fresh parsley. Give it one last toss to combine everything. Taste the pasta and add more salt if needed. Serve immediately, topped with a generous sprinkle of freshly grated Parmesan cheese if you like.