Dice the chicken breast into bite-sized pieces. Chop the onion finely, slice the mushrooms, and mince the garlic cloves. Grate mozzarella and Parmesan cheese if not pre-shredded.
Heat olive oil in a large oven-safe skillet over medium heat. Add diced chicken, season with salt, pepper, and dried thyme. Cook 5-7 minutes until mostly cooked and golden. Remove chicken and set aside.
In the same skillet, sauté chopped onion until translucent, about 3 minutes. Add sliced mushrooms and cook until moisture releases and browns, about 5 minutes. Stir in minced garlic and cook 30 seconds until fragrant.
Return chicken to skillet. Add orzo pasta, chicken broth, and heavy cream. Stir to combine and bring to a gentle simmer. Cook uncovered for 10-12 minutes, stirring occasionally, until orzo is tender and most liquid absorbed.
Stir in half of the shredded mozzarella and all Parmesan cheese. Sprinkle remaining mozzarella evenly over the top. Transfer to casserole dish if skillet isn’t oven-safe.
Bake in a preheated oven at 375°F (190°C) for 15-20 minutes, until cheese is melted and lightly golden.
Remove from oven and let rest a few minutes. Garnish with freshly chopped parsley. Serve warm and enjoy.