Preheat oven to 350°F
Over medium heat, add 2 tablespoons of olive oil to a large skillet.
Using a large dish, combine the flour, salt, and pepper. Working in batches, dredge each cutlet through the seasoned flour shaking off any excess and place into the hot skillet. Brown both sides of the cutlets about 3 minutes per side. Adding another 1-2 tablespoons of olive oil for each batch or as needed. Place browned cutlets in a large baking/casserole dish.
Place the sliced mushrooms in the skillet and cook for about 5 minutes until they release their water and mushrooms are slightly browned. Spoon the cooked mushrooms over the chicken in the baking dish.
Next add your thinly sliced onions to the skillet. If the pan is dry, add a little olive oil. Sautee onions for about five minutes until onions are soft and translucent. To the hot pan with the onions, add the chicken stock and balsamic vinegar. Carefully stir the mixture, loosening any of the brown bits at the bottom of the pan, don’t want to miss getting all that extra flavor. Cook sauce for 1-2 minutes.
Spoon the cooked onions and the sauce over the prepared chicken in the baking dish. *At this point, you could refrigerate the baking dish and hold for cooking.
Place covered baking dish into the preheated oven, bake for 45 minutes or until edges begin to bubble.
Top with the fresh chopped parsley and serve.