If using chicken breasts, butterfly them. If using thighs, you can skip butterflying the meat.

Pat the chicken dry with paper towels and season all sides generously with salt and pepper.

In a large dutch oven, heat the olive oil over medium heat. Once hot, add the chicken (working in batches as needed) and cook until golden brown on both sides and fully cooked through (the internal temperature should read 165°F), 5-7 minutes per side. Remove the chicken to a plate.

Keeping the pan over medium heat, add the onion and cook until softened, about 5 minutes. Add the garlic and herbs and cook for 1 additional minute.

Add the flour to the pot; stir to coat the onion and garlic and cook for about 1 minute before adding the chicken stock and mustard. Stir well to combine, using your spoon to scrape the browned bits from the bottom of the pot as you do. Bring to a simmer.

Add the orzo, cream, parmesan, lemon juice and zest, and tomatoes (if using), along with a heavy pinch of salt and pepper. Stir to combine. Reduce the heat to low; cover and simmer for 8-10 minutes or until the orzo is cooked through, stirring occasionally.

Once the orzo is cooked, taste for seasoning and add additional salt and pepper as needed.

Stir in the spinach and nestle the chicken back to the pot. Allow the spinach to wilt and the chicken to warm back through before serving. Serve with additional parmesan cheese and a squeeze of lemon if desired.