Begin by filling a large pot with water and bringing it to a boil. Add a generous pinch of salt to the water, then add the fettuccine. Cook according to the package instructions until al dente, usually around 8-10 minutes. Stir occasionally to prevent sticking. Once cooked, reserve about 1 cup of pasta water, then drain the fettuccine in a colander.
In a large skillet over medium heat, melt the 4 tablespoons of unsalted butter. Allow it to bubble gently, then add the lemon juice and lemon zest. Stir to combine, letting the mixture cook for about 1-2 minutes until fragrant.
Reduce the heat to low and add the 3 tablespoons of mascarpone to the skillet. Stir continuously until the mascarpone has melted and the sauce is creamy and well combined. If the sauce is too thick, gradually add some of the reserved pasta water until you reach your desired consistency.
Add the drained fettuccine to the skillet with the lemon mascarpone sauce. Gently toss the pasta in the sauce, ensuring every strand is coated beautifully. If needed, add more reserved pasta water to help the sauce adhere to the fettuccine.
Once the pasta is thoroughly coated, remove it from the heat. Serve the Fettuccine with Lemon Mascarpone Sauce immediately, garnished with additional lemon zest or freshly cracked black pepper if desired.
