You can start the pasta and blanch the lemons at the same time, beginning by bringing two pots of water to boil.
Zest two of the lemons. Then, cut all the lemons into quarter lengthwise. Slice the quarters thinly crosswise into triangles.
Cook the pasta to an al dente state. Remove and drain, reserving 1 cup of the cooking liquid.
While the pasta is cooking, add the lemon slices to the second pot of water. Cook for about two minutes. Remove from the water and pat them dry.
Add 1 tablespoon of the oil in a skillet over a high setting. Add the lemon slices, along with a pinch of salt and sugar. Saute until the lemon slices are caramelized – they will be slightly browned at the edges. It should take 3 to 5 minutes. Transfer the lemons to a plate and set aside.
Add 1 tablespoon to a second pan and cook the chopped spinach, adding a splash of water if necessary. Season with salt and pepper and cook until wilted, or about 3 minutes. Transfer to a bowl.
Add 1 tablespoon of oil to a pan at a medium setting. Add the onions, chili flakes, and lemon zest. Cook for 3 to 5 minutes until the onions are translucent and the mixture is fragrant. Stir in the reserved pasta cooking water, cream, sugar, and salt. Cook for an additional 2 minutes.
Now add the pasta, caramelized lemon slices, spinach, and cheese. Toss to coat the pasta thoroughly.
Serve it topped with spoonfuls of ricotta, toasted pine nuts, and more Parmigiano as desired.