Soak the porcini: Put the dried porcini in a bowl. Cover with hot tap water. Let sit 10 minutes. Lift them out and chop fine. Save ½ cup of the soaking liquid.
Cook the chicken: Heat a drizzle of olive oil in a large skillet or wide pot over medium heat. Add the chicken. Season with salt and pepper. Cook 5–6 minutes until lightly browned. Remove and set aside.
Cook the vegetables: In the same pan, add a little more oil if needed. Add the chopped onion. Cook 3–4 minutes until soft. Add the garlic. Stir 30 seconds. Add the sliced baby bella mushrooms. Cook 4–5 minutes until softened.
Add porcini and liquid: Add the chopped porcini to the pan. Pour in the ½ cup reserved porcini soaking liquid. Simmer 1–2 minutes.
Cook the orzo: Stir in the dry orzo. Add the chicken broth and milk. Bring to a gentle simmer. Cover and cook 8–10 minutes, stirring once or twice, until the orzo is tender.
Finish the dish: Return the chicken to the pan. Stir in the Parmesan until creamy. Add spinach if using and stir until wilted, about 1 minute. Taste and adjust salt and pepper.
Serve: Spoon into bowls while hot. Serve as-is or with extra Parmesan on top.
