Boil a large pot of well salted water. Cook spaghetti until just shy of al dente. Reserve one cup of the hot pasta water and drain.
Warm olive oil in a wide skillet over medium low. Add sliced garlic and cook until pale gold. Do not let it brown.
Stir in red pepper flakes, salt, and black pepper. Add a splash of the reserved water to stop the cooking.
Toss in the drained pasta. Add half the reserved water and shake the pan until the sauce clings and looks glossy.
Add lemon zest and juice. Toss again. If the pasta looks tight, add more reserved water a little at a time.
Fold in parsley. Taste and adjust salt and pepper. Finish with cheese if you like and serve right away.