Bring a large pot of generously salted water to a boil. Cook the pasta according to the package timing until just al dente, reserve about 1 cup of pasta water, and drain.

Heat a large skillet or Dutch oven over medium-high heat. Add the sausage, using the olive oil only if the sausage is not very fatty, and cook for 5–7 minutes while breaking it into small pieces until browned and cooked through; drain excess fat if needed.

Add the diced onion and cook for 3–4 minutes until softened. Add the minced garlic and red pepper flakes, if using, and cook for about 30 seconds.

Stir in the tomato paste and cook for 1 minute. Add the diced tomatoes with their juices and the chicken broth, then simmer for 5–7 minutes until slightly reduced.

Lower the heat to medium-low and stir in the heavy cream. Simmer gently for 2–3 minutes, avoiding a hard boil.

Add the drained pasta and toss until coated. Stir in the Parmesan and baby spinach, if using, then cook for 1–2 minutes while adding reserved pasta water a few tablespoons at a time until the sauce is smooth.

Taste and adjust the salt and pepper. Garnish with chopped parsley or basil and additional Parmesan before serving.