In a large pot of salted boiling water, cook the fettuccine according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
In a large skillet over medium-high heat, melt 2 tablespoons of butter. Add the shrimp and season with salt, pepper, and red pepper flakes if using. Cook for 2-3 minutes on each side, or until shrimp are pink and opaque. Remove from skillet and set aside.
In the same skillet, add the remaining 2 tablespoons of butter. Add minced garlic and sauté for about 30 seconds until fragrant. Be careful not to burn the garlic.
Pour in the chicken or vegetable broth, lemon juice, and lemon zest. Stir to combine and bring the mixture to a gentle simmer. If it gets too thick, add a bit of the reserved pasta water until desired consistency is reached.
Add the drained fettuccine and cooked shrimp to the skillet. Toss everything together gently until the pasta is well-coated with the lemon garlic sauce. Cook for an additional minute to heat through.
Plate the fettuccine and shrimp, then sprinkle chopped parsley and grated Parmesan cheese on top for garnish.
Notes
Adjust the red pepper flakes to your heat preference.