Add water to a large pot and add 3 TBSP of kosher salt. Bring to a boil.
Add fettuccine to the water, and cook until just before al dente. Remove 2 cups of the pasta water and set aside. Drain the rest of the pasta water and hold pasta.
Add 1.5 cup of the pasta water to a large skillet. Bring to a simmer and whisk in the butter one TBSP at a time. Make sure the butter is fully incorporated before adding another piece.
Keep whisking and begin adding the Parmesan cheese. Add the other 1/2 cup of pasta water and add the frozen peas. Let simmer for two minutes and then add the pasta. Toss to coat.
Serve hot, topped with parsley, a squeeze of lemon juice and more Parmesan cheese if desired.
0 servings