Bring a large pot of heavily salted water to a rolling boil. Add the spaghetti and cook according to package directions until al dente. Before draining, reserve 1 cup of the pasta cooking water. Drain the spaghetti and set aside.

While the spaghetti cooks, heat the olive oil in a large skillet or deep pan over medium-low heat. Add the minced garlic and red pepper flakes (if using). Sauté gently for 1-2 minutes until fragrant, being careful not to brown the garlic.

Add the lemon juice and 1/2 cup of the reserved pasta water to the skillet. Bring to a gentle simmer. Add the cooked and drained spaghetti directly to the skillet.

Add the butter pieces, lemon zest, and grated Parmesan cheese to the spaghetti. Toss vigorously with tongs, adding more pasta water a tablespoon at a time if needed, until the sauce coats the spaghetti beautifully and emulsifies into a creamy consistency. The butter and cheese should melt and create a luxurious sauce with the lemon juice and pasta water.

Stir in the fresh parsley. Season with salt and freshly ground black pepper to taste. Serve immediately, garnished with extra Parmesan cheese and a sprinkle of fresh parsley.