Step 1: Bring a large pot of salted water to a rolling boil. Add your chosen pasta and cook according to package directions until al dente. Before draining, reserve about 1 to 1.5 cups of the starchy pasta water. Drain the pasta and set aside.
Step 2: While the pasta is cooking, heat a large skillet or Dutch oven over medium-high heat. Add olive oil. Add the sausage meat to the hot pan. Break it up with a spoon and cook, stirring occasionally, until browned and cooked through. Drain any excess grease, leaving just a tablespoon or two for flavor.
Step 3: Reduce the heat to medium. Add the minced garlic and red pepper flakes (if using) to the skillet with the cooked sausage. Sauté for about 1 minute until fragrant, being careful not to burn the garlic.
Step 4: Stir in the butter until it melts and coats the sausage and garlic. Add the fresh spinach, if using, and cook until it wilts, about 1-2 minutes.
Step 5: Add the cooked pasta to the skillet with the sausage mixture. Pour in about 1/2 cup of the reserved pasta water. Toss vigorously, allowing the sauce to emulsify and coat the pasta. Add more pasta water, a tablespoon at a time, until the sauce reaches your desired consistency – creamy and clinging to the pasta.
Step 6: Stir in the grated Parmesan cheese and fresh parsley or basil. Taste and adjust seasonings, adding salt and black pepper as needed. Serve immediately, garnished with extra Parmesan and fresh herbs.