Cook the lentils according to package directions until tender.
Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions for al dente. Reserve about 1 cup of the cooking water, then drain the pasta.
Meanwhile, combine the marinara sauce and cottage cheese in a high-speed blender. Add the cooked lentils and blend until completely smooth.
Pour the sauce into the now-empty pasta pot and and set over low heat. Add just enough pasta water until the sauce reaches your desired consistency. Taste and season with salt, if needed.
Add the cooked pasta and toss until evenly coated. Cook, stirring occassionally, until warmed through, adding more pasta water as needed to loosen the sauce.
