Preheat your oven to 350°F / 180°C. Set a large pot of water over high heat to come to a boil.
Start by squeezing as much moisture as possible from your frozen spinach. I like using a clean kitchen towel for this job (pictured above).
Mix the spinach into the ricotta, and season with salt and pepper. Set aside.
Boil your pasta in a large pot of salted water until al dente. This typically takes about 8-10 minutes, but varies from brand to brand and shape to shape. Drain the pasta, then return to the same pot.
Reserve about ¾ cup of pasta sauce for the baking dish. Add remaining 3 ¼ cups marinara sauce, lentils, and ½ cup of mozzarella cheese to the pasta. Stir to combine.
Pour ¾ cup of reserved marinara sauce into the bottom of your casserole dish, and spread out in a thin layer.
Add about half of the pasta to the dish, and spread out in an even layer.
Dollop heaping tablespoons of the ricotta mixture over the top of the pasta.
Add the remaining pasta, and then the remaining ricotta mixture. Use a spoon to gently tuck the ricotta into the pasta as best as possible.
Scatter the remaining 2 cups cheese over the top of the pasta dish.
Cover the casserole dish with foil (or use an oven-proof lid if it comes with one) and bake for 30 minutes.
Remove the foil, and bake for an additional 10 minutes, until the cheese is brown and bubbly on top.