Make the lemon garlic dressing by combining lemon juice, garlic, olive oil, mustard, oregano, salt, and pepper in a small bowl. Whisk together until blended and set aside.
Using paper towels, pat chicken dry. Trim off excess skin and fat and lightly salt and pepper both sides of the chicken.
Heat oil in oven proof skillet over medium heat. Add chicken skin side down. Cook 12 – 15 minutes until golden brown.
Turn chicken over and cook and additional 5 – 7 minutes. Remove chicken from the pan.
Add the onions and mushrooms to the skillet and cook 2 – 3 minutes. Stir in orzo.
Add spinach and mix well until it starts to wilt. Add chicken stock and thyme.
Add the chicken back to the skillet and spoon the lemon garlic dressing over each piece of chicken and pour the rest over the orzo.
Transfer skillet to the oven. Cook for 20 – 25 minutes.
