Bring a large pot of water to a boil, salt it, add the pasta and stir it.Halfway through cooking, add the spinach, stir, and cook until the pasta is very al dente - that's 2 minutes less than what's written on the package instructions.Make the lemon sauce while the pasta boils.
To a large skillet, add the olive oil, grated garlic, grated lemon zest, and red pepper flakes.Warm it up slowly on low heat, then let the ingredients fry gently for 30 to 60 seconds without burning the garlic.Turn the heat off, add the lemon juice, and wait for the pasta.Tip: Only zest the yellow peel of the lemon, the white pith can be bitter.
When the pasta is ready, save one cup of pasta water, drain the pasta and spinach and add them to the lemon sauce.Add a ladleful (1/2 cup) of pasta cooking water and cook on medium heat, stirring often for a short minute.
Add the grated parmesan, turn the heat off, and stir for 30 more seconds until the cheese melts.Taste and adjust for salt; add more reserved pasta water if the pasta gets too dry.
Serve immediately and optionally sprinkle with parmesan cheese and grated lemon zest.
Nutrition information is for one portion of lemon pasta out of four.*You'll need about 3 quarts (12 cups or 3 Liters) of water and 1 ½ tablespoons (21 grams) of salt to cook 12 ounces (340 grams) of pasta.