In a pan, pour in half a cup of cream and the lemon juice. Stir. Heat until just bubbling. Add the butter. Stir just until melted. Turn off the heat.
Add the cooked pasta to the sauce. Add the Parmesan and toss well. Add the rest of the cream. Toss once more. Add salt and pepper (and lemon zest and mixed herbs, if using) and toss a few more times to blend well.
Serve the pasta Alfredo at once.