Bring a large pot of salted water to a boil for the pasta. Cook according to package directions until al dente. Reserve ½ cup of pasta water before draining.
While the water is coming to a boil, heat a large skillet over medium-high heat. Add the olive oil and Italian sausage, removing it from the casings if necessary. Break it into chunks with a wooden spoon as it cooks.
Once the sausage is browned (about 5-7 minutes), transfer it to a plate using a slotted spoon, leaving the flavorful fat in the pan. Don't rush this step—those caramelized bits are pure gold.
In the same pan, add the diced onion and red bell pepper. Sauté for about 4-5 minutes until they begin to soften.
Add the garlic, Italian seasoning, and red pepper flakes (if using). Cook for just 30 seconds until fragrant. Don't let the garlic burn, or it'll get bitter.
Pour in the white wine or chicken broth, scraping up all the browned bits from the bottom of the pan. Let it reduce by half, about 2 minutes.
Reduce heat to medium-low and add the heavy cream and chicken broth. Let it simmer gently for 2-3 minutes until it starts to thicken slightly.
Return the cooked sausage to the pan along with any accumulated juices. Stir to combine.
Add the Parmesan cheese and stir until melted and the sauce is smooth. If the sauce seems too thick, add a splash of the reserved pasta water.
Toss in the drained pasta and stir to coat in the sauce. Let it cook for a minute to absorb some of the flavor.
Add the chopped spinach and stir until it wilts, which should take just a minute or two.
Taste and adjust seasonings with salt and pepper. Remember that Parmesan and sausage are already salty, so go easy on the additional salt.
Serve immediately, garnished with extra Parmesan cheese and fresh herbs.