Place the greens, garlic, pine nuts, sea salt and pepper, and half the Parmesan in a food processor. Pulse while adding the olive oil, little by little, until the mixture becomes a paste.
In a saute pan, add enough oil to coat the pan. Heat to medium-high. Add the sausage and cook through for 10-to-12 minutes.
Add onions and then mushrooms. Cook for another 8-10 minutes. Let simmer.
Meanwhile, cook the pasta according to directions for al dente. With a slotted spoon, add the cooked pasta to the pan with the sausage. Cook for about 3 minutes. Add the pesto, a little at a time, until the pasta is nicely coated.
Plate with a sprinkle of remaining Parmesan, and, if you like, red pepper flakes.