Bring 4 quarts of water to a boil in a large saucepan or Dutch oven. Add pasta to pan, and cook 8 minutes or until almost al dente. Drain pasta in a colander over a bowl, reserving 3/4 cup cooking liquid. Discard the remaining cooking liquid.
Heat a large heavy skillet over medium-high heat. Add Italian sausage to pan; sauté 6 minutes or until browned, stirring to crumble. Remove sausage from pan, reserving drippings; drain on paper towels. Add garlic to drippings in pan; sauté 1 minute, stirring constantly. Add 1/2 cup reserved cooking liquid to pan; bring to a boil, scraping pan to loosen browned bits. Stir in 2 tablespoons olive oil; cook 30 seconds, stirring constantly with a whisk. Add pasta to pan; cook 1 minute, tossing to combine. Remove from heat.
Combine about 2 tablespoons cheese and 4 3/4 cups parsley leaves in a food processor; process until finely ground. With motor running, add remaining 1/4 cup cooking liquid and remaining 2 tablespoons olive oil; process until smooth. Add parsley mixture and salt to pasta; toss well to coat. Divide pasta mixture evenly among 4 shallow bowls; top evenly with sausage, remaining 2 tablespoons cheese, and remaining 1/4 cup parsley leaves. Serve immediately.