Heat water for the pasta in a large stockpot.

Heat up oil in a large skillet over medium heat. Add the sausage and crumble it with a fork. Brown it until it loses the raw red color and starts browning, creating brown bits on the bottom of the pan. Season with black pepper and salt unless your jar of basil pesto lists salt as one of its ingredients. Toss in fennel seeds and sliced onion and Sauté for several minutes until the onion turns translucent.

Your pasta water should be boiling by now, so toss in the pasta and cook according to the instructions on the box.

Lower the heat on the skillet and let the sausage cook on low heat (stirring occasionally) until the pasta is ready.

Before draining the pasta, dip out 1-2 cups of pasta water and set aside. Drain pasta.

Dissolve 1 tbsp of basil pesto in 1/2 cup of pasta water and slowly stir into the sausage mix. Stir well and scrape any brown bits off the bottom of the skillet into the sauce. Add the remaining basil pesto to the skillet and combine well. Slowly pour in another 1/2 cup of pasta water and let the sauce thicken. Toss in the drained pasta and salt for taste. Add a few tablespoons of pasta water, if needed.

Sprinkle in cheese, basil leaves or fennel seeds and serve hot.