In a large pot with boiling salted water, add the spaghetti and cook according to the package directions minus 1 minute.
Meanwhile, in a large cold skillet, add oil; set the heat on medium. While it’s heating up, add pressed garlic and stir very well before adding parsley stems and half red pepper flakes.
Stir the ingredients and let the oil mixture infuse for about 5 minutes, stirring often – don’t burn the garlic. If the heat is too high, reduce to medium-low and don’t let the mixture simmer too much.
After 5 minutes, add a couple ladles of the pasta water and give a good stir. Simmer gently for 2 minutes.
Remove the parsley stems and discard them.
When the pasta is almost done, transfer it to the skillet and reserve ½ cup of pasta water on the side.
Bring the heat back to medium and gently toss the noodles until nicely coated.
Add remaining red pepper flakes and cook for 2 minutes. If not done, add more pasta water.
When pasta is cooked, sprinkle on the finely chopped parsley; toss to blend.
Transfer to serving plates and sprinkle on more red pepper flakes.