Cook the spaghetti in a large pot of boiling salted water.
Meanwhile, zest and juice the lemon.
Prepare the sage butter.In a skillet, melt the butter. Add the sage leaves and simmer for 1 minute.Finely chop the sage leaves, if desired.
Add 4 tablespoons of pasta cooking water and the zest. Stir and simmer over low heat.
Drain the al dente spaghetti (keeping a little of the cooking water) and add it to the pan with the sage butter.Add 1 or 2 tablespoons of lemon juice, the Parmesan cheese, and a splash of cooking water.Mix well until you get a smooth, creamy paste.
Divide the spaghetti among plates, season with pepper, and enjoy immediately!You can add a little zest and Parmesan cheese.
Notes
For this recipe, I added Parmesan cheese to the sage, but you can leave it out, or simply sprinkle it over the dish when serving.