Cook the pasta in salted, boiling water until the spaghetti is al dente (has a little chew), and then drain it. Return the drained pasta immediately to the same cooking pot, off the heat. Quickly add the olive oil and lemon mixture, grated fresh Parmesan, and a sprinkling of red pepper flakes, if using. Mix the pasta and sauce well to combine and so the cheese coats the strands. Taste it to see if it needs anything more, like salt, pepper, cheese, or lemon juice. Toss it again. Plate the pasta and on each serving sprinkle on the reserved parsley and the remaining lemon zest.