Bring a large pot of salted water to a rolling boil. Add the pasta, stirring gently. Cook according to package directions until al dente, about 8-10 minutes. Reserve a few tablespoons of pasta water, then drain.

In a large skillet, heat over medium-high heat. Add Italian sausage and cook for 8-10 minutes until browned. Transfer to a paper towel-lined plate.

In the same skillet, add olive oil. Add onion and both bell peppers, stirring occasionally for 7-8 minutes until softened and lightly browned.

Lower the heat to medium-low, add butter and garlic, stirring until butter melts. Sprinkle in flour, stirring for another minute. Gradually add cream and mustard, stirring until the sauce thickens slightly.

Return sausage to the skillet and add spinach. Toss everything gently to allow spinach to wilt and absorb flavors.

Fold the drained pasta into the mixture, adding reserved pasta water to reach desired consistency. Serve hot, garnished with salt, pepper, and Parmesan cheese.