Season the chicken: Pat chicken dry. Season generously with salt, pepper, and half the Italian seasoning.
Sear the chicken: Heat olive oil in a large deep skillet over medium-high. Add chicken in a single layer and cook until lightly browned and just cooked through, 4–6 minutes.
Transfer to a plate.
Sauté aromatics: Reduce heat to medium. Add butter and onion. Cook until softened, 3–4 minutes.
Stir in garlic, remaining Italian seasoning, and red pepper flakes; cook 30 seconds until fragrant.
Toast the orzo: Add orzo to the skillet. Stir to coat in butter and aromatics, and toast for 1–2 minutes until slightly golden. This boosts flavor and helps the orzo stay pleasantly firm.
Add broth and lemon zest: Stir in chicken broth and lemon zest.
Bring to a gentle simmer. Scrape the bottom of the pan to release browned bits for extra flavor.
Cook the orzo: Simmer, stirring occasionally, until orzo is al dente, 9–11 minutes. If it looks too dry before the orzo is tender, add splashes of hot water or more broth.
Finish the sauce: Lower the heat.
Stir in heavy cream and Parmesan until melted and creamy.
Add chicken back in: Return the chicken and any juices to the skillet. Stir to combine and warm through for 1–2 minutes.
Add lemon juice and greens: Stir in lemon juice to taste. Fold in spinach until just wilted, about 30–60 seconds.
Taste and adjust salt, pepper, and lemon.
Serve: The sauce should be creamy and loose, not soupy. If too thick, add a splash of warm water or broth. Garnish with parsley and extra Parmesan.
Serve hot.