Cut each chicken breast into thirds by cutting each breast in half lengthwise first, then setting the thinner half aside. Working with the thick part of the breast, slice in half horizontally to produce 2 more thin cutlets. Cover chicken breast pieces with a sheet of plastic wrap and pound with a meat mallet or the bottom of a small skillet until all of the pieces are about ¼" thick. (see how)

Season chicken cutlets on both sides using salt and pepper then sprinkle tops with garlic powder and dried oregano. Dredge each piece of chicken in flour then shake to remove any excess.

Place a large skillet over medium-high heat then add 3 Tbsp unsalted butter. Place as many cutlets as will comfortably fit into skillet and cook until golden brown—about 3 minutes per side. The chicken may not be fully cooked at this point, but it will finish cooking in the sauce. Transfer to a plate then repeat until all chicken is browned.

Next, place 2 Tbsp unsalted butter into skillet along with all the bits leftover from browning the chicken and allow it to melt. Add sliced mushrooms then cook, stirring frequently for about 5 minutes until tender and golden brown around the edges.

Add Marsala wine then bring to a boil, scraping to remove any bits from the bottom of the pan. Allow wine to bubble away for 4–5 minutes until wine has reduced by half. Add chicken stock then return to a boil. Reduce heat to medium-low and simmer for 5 minutes, just to reduce sauce and allow flavors to blend.

Return chicken cutlets to pan, turn to coat in sauce and allow to simmer for about 5–6 minutes until chicken is cooked through and breading from chicken has thickened the sauce. Sprinkle with fresh parsley then serve.