Cook the pasta according to package instructions in a large pot of salted boiling water until al dente. Reserve 1/2 cup of pasta water, then drain the pasta.
In a large skillet over medium heat, melt the butter. Add the minced garlic and red pepper flakes, if using. Sauté for about 1-2 minutes until fragrant, being careful not to burn the garlic.
Add the drained pasta to the skillet, along with the reserved pasta water, salt, and black pepper. Toss to combine and coat the pasta in the garlic butter sauce.
Remove from heat and stir in the grated Parmesan cheese, chopped parsley, and lemon juice, if using. Toss until everything is well combined and the cheese is melted.
Serve immediately, garnished with additional parsley and Parmesan cheese if desired.