In a large skillet over medium heat, add the Italian sausage and cook for 3–4 minutes, breaking it up with a wooden spoon until browned. Once the sausage is nearly cooked through, toss in the diced onion to soften.

Lower the heat to medium-low and add the minced garlic along with the dried marjoram, cracked black pepper, crushed rosemary, dried thyme, and red pepper flakes. Stir well, cooking for another minute until fragrant, then remove the sausage mixture from the pan and set it aside.

In a separate pot, bring salted water to a boil, then add the rigatoni. Cook until al dente, about 10–12 minutes, stirring occasionally to prevent sticking. Once ready, drain the pasta and reserve a cup of the pasta water.

Using the same skillet, pour in the chicken broth and bring it to a boil over medium heat, scraping up any flavorful bits stuck to the pan. Once boiling, stir in the heavy cream and simmer until it thickens slightly.

Lower the heat and stir in the grated Parmesan cheese, sun-dried tomatoes, and baby spinach. Continue stirring until the cheese melts and the spinach wilts.

Return the cooked sausage and drained rigatoni to the skillet, tossing everything together until evenly coated in the creamy sauce. Heat through for another 2–3 minutes, adding reserved pasta water as needed for desired consistency.