Brown the sausage: Heat the olive oil in a large skillet over medium heat. Add the sliced chicken sausage and cook 3–4 minutes, turning once, until lightly browned.
Transfer to a plate and keep warm.
Sauté the aromatics: In the same skillet, add the onion and a pinch of salt. Cook 3–4 minutes until softened. Stir in the garlic and cook 30 seconds until fragrant.
Deglaze and simmer: Pour in the chicken broth, scraping up any browned bits.
Stir in the Italian seasoning and crushed red pepper flakes, if using. Bring to a gentle simmer.
Add the gnocchi: Stir in the gnocchi and tomatoes. Cover and cook 3–4 minutes, stirring once or twice, until the gnocchi is tender and floating and the tomatoes begin to soften.
Make it creamy: Reduce the heat to low.
Stir in the heavy cream and Parmesan. Simmer 2–3 minutes, stirring often, until the sauce thickens slightly.
Fold in greens and sausage: Add the spinach and the browned sausage. Cook 1–2 minutes, just until the spinach wilts and the sausage is heated through.
Taste and season with salt and black pepper.
Finish and serve: Remove from heat. Let the skillet sit for a minute to thicken. Top with extra Parmesan and chopped basil or parsley.
Serve hot.