If using chicken breasts, butterfly them for even thickness; if using thighs, leave whole.
Pat the chicken dry and season all sides generously with salt and pepper.
Heat the olive oil in a large Dutch oven over medium heat. Add the chicken in batches if needed and cook until golden and cooked through, about 5–7 minutes per side (165°F internal temperature). Transfer cooked chicken to a plate.
With the pan still over medium heat, add the diced onion and cook until softened, about 5 minutes.
Add the minced garlic, dried thyme, and Italian seasoning; cook 1 minute until fragrant.
Sprinkle the flour over the onion mixture and stir to coat; cook about 1 minute.
Slowly pour in the chicken stock and add the Dijon mustard, scraping any browned bits from the bottom of the pot. Bring to a simmer.
Stir in the dry orzo, heavy cream, grated Parmesan, lemon juice, lemon zest, and tomatoes (if using). Season with a generous pinch of salt and pepper.
Reduce heat to low, cover, and simmer, stirring occasionally, until the orzo is tender and most liquid is absorbed, about 8–10 minutes.
Taste and adjust seasoning with salt and pepper as needed.
Stir in the baby spinach and nestle the cooked chicken back into the pot; allow the spinach to wilt and the chicken to warm through before serving.
Serve warm, topped with additional Parmesan and a squeeze of lemon if desired.
