In a large ziploc bag add chicken breast and seal the bag. Using a mallet pound chicken on both sides to thin them out a bit. Add 1/2 cup of arrowroot starch, salt, and pepper. Shake the bag to coat the chicken all over.
In a large pan over medium/high heat drizzle oil to coat the bottom. Add your chicken breasts to the pan and brown on both sides. This usually takes about 3-4 minutes on each side. When chicken has been browned place on a large plate and set aside.
Reduce heat to medium and add shallots to the pan, saute for 1-2 minutes. When shallots have been softened add garlic and mushrooms, saute for another minute. Next, add marsala wine and bring to a boil. Let wine reduce a bit then add chicken stock and oregano. Bring sauce to a boil over medium heat and add chicken back to the pan. Place a lid on the pan and reduce heat to medium/low. Cook the chicken for 15-20 minutes.
When chicken is done cooking remove and place on a new clean plate. It's time to thicken the sauce. Mix arrowroot starch/flour and the 2 tbsp of melted butter. Pour mixture into your sauce and bring to a boil. Let sauce thicken for a few minutes. Add chopped parsley to the sauce and mix well. Taste and add salt/pepper if needed.
To plate, add chicken to your favorite pasta or mashed potatoes and then spoon sauce over top. Garnish with more fresh parsley.