Cook the pasta - Bring a large pot of salted water to a boil. Add the Mafaldine pasta to the boiling water and cook, stirring occasionally, until al dente, about 8-10 minutes. Drain the pasta, reserving 1 cup of pasta cooking water, and set aside. - Be sure to salt the water generously before cooking the pasta. The salty water will help flavor the pasta and bring out its natural flavors. Additionally, be sure to reserve some of the pasta cooking water to use later if the sauce needs thinning. The starchy water will help thicken the sauce and make it silkier.

Cook the sausage - Heat a large skillet over medium-high heat. Add the fennel sausage to the skillet and cook, breaking it up with a wooden spoon, until browned and crisp, about 8-10 minutes. - Be sure to use a good quality fennel sausage to get the best flavor possible. When cooking the sausage, try not to over-crowd the skillet, or it may take longer to brown and crisp up. You want to achieve a nice brown color on the sausage for maximum flavor.

Cook the leek ragu - Add the butter to the same skillet and melt over medium heat. Add the leeks and garlic to the skillet and cook, stirring occasionally, for 5 minutes until the leeks are soft. Add the white wine to the skillet and deglaze, scraping up any browned bits from the bottom of the pan. Cook for 2 minutes or until the wine has almost evaporated. Add the chicken stock, tomato paste, salt, and black pepper to the skillet and stir well to combine. Bring the mixture to a simmer, reduce the heat to low, and let the ragu cook for 20-25 minutes, or until the liquid has reduced by half. - Be sure to clean the leeks thoroughly before cooking. Leeks often have dirt and sand in the layers, so it's important to rinse them well under running water and pat them dry before cooking. When deglazing the skillet with white wine, be sure to scrape up any browned bits from the bottom of the pan. Those bits are full of flavor and will add depth to the sauce.

Combine the pasta and sauce - Add the cooked pasta to the skillet with the leek ragu and toss well to coat. Add some reserved pasta water if the sauce seems too thick. Stir in the grated Parmesan cheese and toss well to combine. Serve the pasta hot with extra grated Parmesan cheese on top. - When combining the pasta and sauce, be sure to reserve some pasta water to use if the sauce needs thinning. The starchy water will help thicken the sauce and make it silkier. Be sure to toss the pasta and sauce together well to ensure that every strand of pasta is coated in the rich and velvety sauce.