Bring a large pot of salted water to a boil and cook the pasta until just al dente. Reserve 1 cup of the pasta cooking water, then drain the pasta and set aside.

While the pasta cooks, pat the chicken dry and season both sides with salt and pepper. Heat the olive oil in a large skillet over medium-high heat.

Add the chicken to the pan and cook for 4–5 minutes per side, until golden and cooked through. Transfer to a plate and slice into bite-sized pieces.

Reduce heat to medium. In the same skillet, add the butter if using and sauté the onion until translucent, about 3 minutes.

Add the garlic and cook another 30 seconds until fragrant.

Pour in the chicken broth and scrape any browned bits from the bottom of the pan. Let the broth simmer for 2 minutes to reduce slightly.

Stir in the cream and Dijon mustard, then gently simmer for 3–4 minutes until the sauce thickens.

Add the Parmesan in batches, stirring until smooth and silky. If the sauce gets too thick, add a splash of the reserved pasta cooking water.

Add the spinach and let it wilt into the sauce. Return the sliced chicken and cooked pasta to the pan. Toss gently to coat everything in the sauce.

Taste and season with salt, pepper, and red pepper flakes if using. Finish with a sprinkle of lemon zest and fresh parsley or basil.

Serve immediately with extra Parmesan on the side.