Bring a large pot of salted water to a rolling boil, then add the spaghetti and cook until just al dente, about 8 minutes. Drain, reserving ½ cup of pasta water, and set aside.
While the pasta cooks, heat a skillet over medium-low heat and add the olive oil, warming it until it shimmers gently.
Slide the thinly sliced garlic into the hot oil, cooking slowly and stirring gently until the slices turn a golden, fragrant hue—about 2 minutes. You should hear a gentle crackle and smell a nutty aroma.
Add the red chili flakes to the skillet and stir quickly for about 30 seconds, allowing their spicy fragrance to infuse into the oil and garlic.
Immediately toss the drained spaghetti into the skillet, using tongs or a pasta fork to coat the noodles evenly with the garlic and chili-infused oil. If it looks dry, add a splash of the reserved pasta water and stir until glossy.
Cook everything together for 1-2 minutes, allowing the flavors to meld and the pasta to soak up the fragrant oil. The garlic should be golden and fragrant, and the noodles glossy and coated beautifully.
Remove from heat, taste, and adjust salt as needed. Toss in chopped parsley if using for a bright herbal finish, then transfer to serving plates.
Finish with an extra drizzle of olive oil or a squeeze of lemon if desired, and serve immediately while hot and fragrant.
