1. Heat the olive oil in a large deep skillet over medium-high heat. Add the sliced chicken apple sausage in a single layer and cook for 3 to 4 minutes per side, until deeply browned and caramelized. Transfer the sausage to a plate and set aside, leaving the drippings in the pan.

2. Reduce the heat to medium. Add the diced red onion to the skillet and cook for 3 to 4 minutes, stirring often, until softened and translucent. Stir in the minced garlic and cook for 30 seconds, just until fragrant.

3. Pour in the chicken broth and heavy cream, scraping up any browned bits from the bottom of the pan. Whisk in the Dijon mustard, Italian seasoning, and crushed red pepper flakes if using. Bring the mixture to a gentle simmer.

4. Stir the dry pasta into the simmering liquid, making sure it is mostly submerged. Reduce the heat to medium-low, cover the skillet, and cook for 10 to 12 minutes, stirring every few minutes, until the pasta is al dente and most of the liquid has been absorbed into a creamy sauce. If the pan looks too dry before the pasta is tender, add a splash of extra broth or water.

5. Add the chopped baby spinach to the skillet and stir until it wilts into the hot pasta, 1 to 2 minutes. Return the browned sausage and any accumulated juices to the pan and toss to combine.

6. Remove the skillet from the heat. Sprinkle in the grated Parmesan cheese and stir until it melts into the sauce and coats the pasta. Season to taste with salt and black pepper.

7. Top the pasta with chopped fresh parsley or basil. Serve hot, with extra Parmesan on the side if desired.