If using chicken breasts, butterfly them for even thickness; if using thighs, leave whole.
Pat the chicken dry and season generously on both sides with salt and pepper.
Heat the olive oil in a large Dutch oven over medium heat. Add the chicken (work in batches if needed) and cook until golden and cooked through, about 5–7 minutes per side; use an instant-read thermometer to confirm 165°F. Transfer 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 and cook 1 minute more.
Sprinkle the flour over the onion mixture and stir to coat; cook for about 1 minute.
Slowly add the chicken stock and Dijon mustard, scraping up any browned bits from the bottom of the pot. Bring the mixture to a simmer.
Stir in the dry orzo, heavy cream, grated Parmesan, lemon juice, lemon zest, and halved tomatoes (if using). Season with a generous pinch of salt and pepper, then reduce heat to low.
Cover and simmer gently, stirring occasionally, until the orzo is tender and most liquid is absorbed, about 8–10 minutes.
Taste and adjust seasoning with salt and pepper if needed.
Stir in the baby spinach until wilted, then nestle the cooked chicken back into the pot to warm through before serving.