In a medium pot, add lentils, a few pinches of salt and fill with enough water to cover by 2-inches. Bring to a boil over high heat, then reduce the heat to medium low to maintain a rapid simmer for about 20 minutes or until lentils are all the way cooked through and beginning to break down a bit (times will vary depending on freshness).
Meanwhile, fill a large pot with about 3 quarts of water and bring to a boil. Add a few generous pinches of salt along with your pasta. Cook the pasta according to the package instructions. Reserve at least 1 ½ cups of the pasta cooking water, then drain and transfer back to the pot.
To a high-speed blender, add 1 ¼ cup of the cooked lentils and transfer the rest to a medium bowl. To the blender, add the cheese, ¼ cup oil, 2 tablespoons lemon, garlic, ½ teaspoon salt and 1 cup of the pasta water. Blend on high until totally smooth, about 1 minute.
In the bowl with the remaining lentils, stir in remaining ½ teaspoon salt, 1 tablespoon of lemon juice, and 1 tablespoon olive oil. Taste and add more lemon, if you’d like (I like it very lemony!). Set aside.
Pour the lentil sauce over the pasta and toss to coat. If the sauce is looking dry or gloppy (this can happen if it sits out too long as well), add more pasta water and keep tossing (or if you run out, just use regular warm or hot tap water). Taste and season with salt and pepper.
Serve right away and top with a generous spoonful of the reserved lentils, sausage (if using), more parmesan, nuts, herbs, pepper, and a drizzle of olive oil.