Brown the sausage. Heat your pot or deep skillet over medium-high heat. Add the Italian sausage, breaking it apart with a wooden spoon as it cooks until nicely browned, about 5-7 minutes.
Add aromatics. Reduce heat to medium and add diced onion to the pot with the sausage. Cook for 2-3 minutes until onion begins to soften. Add garlic and bell pepper (if using) and cook for another minute until fragrant.
Season the base. Stir in Italian seasoning and red pepper flakes (if using). Cook for 30 seconds to bloom the spices, which releases their flavors.
Build the sauce. Add diced tomatoes with their juice, stirring to combine and scraping any browned bits from the bottom of the pot (that's flavor gold!).
Add liquids and pasta. Pour in the chicken broth and heavy cream, stirring to combine. Add the dry pasta and stir well to ensure it's submerged in liquid.
Bring to a perfect simmer. Increase heat to medium-high and bring the mixture to a boil. Once boiling, reduce heat to maintain a gentle simmer. Cover the pot.
Cook pasta to perfection. Simmer covered for about 10-12 minutes, stirring occasionally to prevent sticking. The pasta should be al dente and most of the liquid absorbed, but some sauce should remain. If it seems too dry, add a splash more broth; if too wet, simmer uncovered for a few more minutes.
Final touches. Remove from heat and stir in the shredded cheese until melted. If using spinach, stir it in now to wilt from the residual heat.
Rest briefly. Let the pot stand covered for 2-3 minutes to allow the sauce to thicken slightly and flavors to meld.
Serve with flair. Garnish with fresh basil if using, and serve directly from the pot for true one-pot meal simplicity.