Coarsely chop the basil leaves. Pulse the chopped basil and 2 tablespoons olive oil in food processor until the pieces of basil are very small. Add flour and eggs and turn processor on until the dough comes together into one ball (about 30 seconds). The dough should barley stick to the sides of the food processor. Add a couple drops water if the dough is too dry. Add 1/2 teaspoon flour if dough is too wet. Cover dough ball with plastic wrap and let rest for at least 20 minutes. Dough can sit in refridgerator up to 24 hours.
Remove casing from sausage. Cut into 1/2 inch pieces. Cook sausage in pan over medium heat until just cooked through, about 6-8 minutes. Set aside on paper towels.
Roll pasta using a pasta roller with the cutter set to fettucine.
Bring 4-quarts water to full boil. Salt water liberally with kosher salt before adding pasta (do not skip this step). Cook pasta until just al-dente, about 3 minutes for fresh fettucine.
Cook the onions with 3 tablespoons extra virgin olive oil until translucent over medium heat in large sauté pan, about 8 minutes. Add sausage, garlic, and herbs. Cook until garlic begins to turn slightly brown, about 1-2 minutes. Remove pan from heat and add cooked pasta. Toss the pasta with the sausage and onion mixture. Season with freshly ground black pepper. Sprinkle grated parmesan on top. Garnish with thinly sliced basil and serve immediately.