Bring a large pot of salted water to a boil. Add in the pasta of your choice and cook until al dente.
At the very last minute, right before draining the pasta, add in the spinach and cook for 30-45 seconds. Reserve 1 cup of pasta cooking water and drain the pasta.
Put the now-cooked pasta back in the pan.
Meanwhile, make the ricotta lemon pasta sauce by mixing ricotta cheese, parmesan cheese, garlic, lemon zest, lemon juice, and salt and pepper in a bowl.
Add in the ½ cup of reserved pasta sauce and whisk until creamy.
Add the lemon ricotta sauce to the pasta and give it a gentle toss. It should be very creamy as you toss it with the help of the residual heat coming from the freshly cooked pasta. If you need it, add in the rest of the reserved pasta liquid and turn the heat on for a minute or so as you toss and turn the pasta.
Taste for seasoning and add in if necessary. Garnish with a sprinkle of shredded parmesan cheese and a drizzle of olive oil and red pepper flakes, if using. Serve.