How to Make It
In a large pot of generously salted boiling water, cook the pasta until just al dente. Drain the pasta, reserving 1 cup of the pasta cooking water.
Meanwhile, in a mini food processor, combine the scallions, parsley, pine nuts and 1/4 cup of the olive oil and process until pureed. Add the 1/2 cup of the cheese, season with salt and pepper and process the pesto just until blended.
In a large skillet, heat the remaining 1 tablespoon of olive oil. Add the sausage and cook over high heat, breaking up the meat with a wooden spoon, until it is lightly browned and no trace of pink remains, about 5 minutes. Drain the sausage and pour off any fat in the skillet.
Return the sausage to the skillet. Add the scallion pesto, pasta and 3/4 cup of the pasta cooking water and simmer over moderate heat, tossing, until a thick, creamy sauce forms, about 2 minutes; add more of the pasta cooking water if the sauce is dry. Transfer the pasta to bowls and serve, passing extra Parmigiano-Reggiano cheese on the side.