1. In a medium skillet on low heat, cook the sausage, breaking it up with a wooden fork into small pieces. Saute until lightly browned, about 5 minutes. If you're using homemade sausage, you may have to add a small amount of olive oil; if it's store-bought, you'll have to remove some of the rendered fat. When the sausage is brown, add the fennel seeds and mix. Keep warm.
2. Beat the eggs in a medium bowl with a wire whisk until slightly thick.
3. Cook the pasta until al dente; do not overcook. Drain well and return the pasta to the pot in which it cooked. Add the butter and toss well. Add the beaten eggs and very quickly keep tossing. Add the sausage with fennel, the cheese, and salt to taste. Add fresh parsley on top of pasta and serve at once. Pass additional cheese.