Bring a large pot of heavily salted water to a boil. Add the pasta to the boiling water and set a timer for 2 minutes less than the cooking time recommended by the manufacturer.

While the pasta is cooking, set a large nonstick pan over medium heat. Add the heavy cream, then very finely grate the zest of the lemon into the cream. Mix the zest into the cream and bring it to a gentle simmer. Start incorporating the butter, 1 or 2 cubes at a time, whisking to incorporate the butter as it melts.

Once all the butter is incorporated, lower the heat and start sprinkling the parmesan over the sauce. Add the cheese very gradually and keep whisking to melt and incorporate the cheese into the sauce.

When the pasta timer goes off, use tongs to transfer the pasta to the pan containing the creamy lemon sauce. It’s okay if some water comes along with it.

Return the saucepan to medium heat. Add ½ cup pasta water to the pan and stir to combine. Bring the sauce back to a simmer and keep it running for 1-2 minutes, until the sauce thickens around the pasta just enough to coat it.

Squeeze the juice from the zested lemon and stir it into the dish. Season with a very generous amount of black pepper.

If using, stir the baby spinach and protein into the pasta and cook for a few seconds more, just until the spinach loses its volume. Add a bit more pasta water to keep things saucy. Garnish with fresh basil, if desired. Serve immediately.