Cut the lemon in half and squeeze out two tablespoons of juice into a small bowl and put to one side.
Cook the pasta in a large pan of boiling well-salted water, stirring occasionally, until very al dente (the pasta will finish cooking in the sauce).
Meanwhile, add the cream to a saucepan with three quarters of the lemon zest and whisk over a medium heat for a couple of minutes until just beginning to simmer. Remove from the heat and whisk in the butter a tablespoon at a time until melted and emulsified, then season to taste with salt and plenty of freshly ground black pepper.
Drain the pasta, reserving a cup of the pasta cooking liquid.
Add the drained pasta to the saucepan with the cream sauce and stir well, adding half the Parmesan until very well combined.
Stir in the lemon juice and add a little pasta cooking water until you have a good consistency to the sauce.
Divide the pasta between four bowls, then top with remaining lemon zest and Parmesan and a little more black pepper. Sprinkle with the walnuts and serve immediately.
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