Combine diced cooked chicken, orzo pasta, and chicken broth in the baking dish, stirring until the pasta is fully submerged.

Heat olive oil in a large skillet over medium-high heat until shimmering.

Add sliced mushrooms and cook while stirring occasionally until they release moisture and begin to turn golden brown.

Stir in the sliced shallots and cook until softened and fragrant.

Add minced garlic, salt, pepper, and dried thyme, stirring briefly until the aroma becomes strong and savory.

Pour in Marsala wine and allow it to simmer for a minute or two until slightly reduced.

Add half-and-half cream and stir gently until the sauce becomes lightly thickened and silky.

Transfer the mushroom sauce mixture into the baking dish with the chicken and pasta.

Stir everything together so the sauce evenly coats the ingredients.

Cover the dish tightly with foil and place it in the oven.

Bake until the pasta becomes tender and most of the liquid has been absorbed.

Remove the foil and sprinkle Parmesan cheese and fresh parsley over the casserole.

Let the casserole rest for a few minutes before serving so the sauce settles and thickens slightly.