Bring a large pot of water to a rolling boil. Add a generous amount of salt (about 1 tablespoon). Add the spaghetti and cook according to package directions until al dente.

While the pasta is cooking, add the olive oil and thinly sliced garlic to a large, cold skillet. Turn the heat to medium-low. Cook slowly, swirling the pan occasionally, for 5-7 minutes, until the garlic is fragrant and turns a pale golden color. Add the red pepper flakes during the last minute of cooking. Be careful not to let the garlic burn, as it will become bitter.

Just before the pasta is finished cooking, use a heatproof measuring cup to reserve at least 1 cup of the starchy pasta water. Drain the pasta in a colander.

Immediately transfer the drained spaghetti to the skillet with the garlic and oil. Increase the heat to medium. Add about 1/2 cup of the reserved pasta water and the chopped parsley.

Toss vigorously and continuously using tongs for 1-2 minutes until the oil and water have emulsified into a light, creamy sauce that coats every strand of pasta. If the pasta seems dry, add another splash of pasta water until it reaches the desired consistency.

Remove from heat. Taste and season with additional salt if needed. Serve immediately, garnished with extra fresh parsley and a sprinkle of Parmesan cheese, if desired.