In a large pot of salted boiling water, cook the spaghetti according to package instructions until al dente. Reserve 1/2 cup of the pasta water, then drain the spaghetti.
In a large skillet over medium heat, melt the butter. Add the minced garlic and sauté for about 1-2 minutes until fragrant but not browned.
Stir in the red pepper flakes and cook for an additional 30 seconds, allowing the heat to develop.
Add the drained spaghetti to the skillet. Toss well to coat the pasta in the garlic butter mixture.
Gradually sprinkle in the grated Parmesan cheese while tossing the spaghetti. If the mixture is too thick, add reserved pasta water a little at a time until the desired consistency is reached.
Season with salt and pepper to taste. If you're using lemon zest, sprinkle it in now for added brightness.
Remove from heat and fold in the fresh chopped parsley. Toss everything together until well combined.
Notes
Serve in warm bowls, garnished with extra Parmesan and parsley. Add a lemon wedge on the side for freshness!