Stir in the rest of the butter, plus the garlic and Dijon mustard. Cook for about a minute.
Add in the broth and lemon juice + zest, and let it bubble for about a minute or so.
Take the pan off the heat and stir in the parmesan cheese and parsley. Add a splash of the hot pasta water prior to draining the pasta (a couple tablespoons) and then toss the pasta with the sauce. Season with salt & pepper as needed and serve immediately.
Notes
For the best flavor, use fresh parsley and high-quality Parmesan cheese. Adjust the seasoning to your taste preferences.