Warm the oil, garlic, and red pepper flakes in a small saucepan over medium-low heat. Cook for 10 to 15 minutes, or until the garlic begins to sizzle and turn golden. Swirl it in the pan a few times before removing it from the stove to let it continue infusing.

If the garlic burns, begin again.

Bring a large pot of water to a boil and salt it liberally before sliding the pasta in. Cook for 7 to 8 minutes, or until al dente. Drain, reserving 1/2 cup of the starchy cooking water. Pour the pasta back into the pot and drizzle in the reserved oil, discarding the garlic cloves. Toss to combine. Add the parsley, lemon zest, Parmesan cheese, a sprinkle of salt, and a few grinds of pepper. If the pasta seems dry after adding the cheese, add the reserved cooking liquid, 1 tablespoon at a time, and toss to coat. The spaghetti should glisten a bit, but it should not be dripping with oil. Serve with additional cheese grated on top, if desired.