Prepare the chicken by slicing the breasts in half horizontally to create four thinner cuts of chicken.
Whisk together the egg and heavy whipping cream in a bowl until well combined.
Combine the flour, breadcrumbs, garlic powder, oregano, onion powder, salt, and pepper in a separate bowl.
Next, working with one cutlet at a time, coat the chicken in the egg mixture, and then allow any excess liquid to drip off before moving on to the next step.
Dredge the chicken in the flour mixture, coating it thoroughly on all sides.
Heat the olive oil in a large skillet over medium-high heat. Sear the chicken for 4-5 minutes on each side. Cook until golden brown and no longer pink. Once the chicken is cooked, remove it from the pan and set it aside.
Gravy
Remove any burnt bits from the bottom of the pan, then add the onion and mushrooms to the pan. Saute until the onions are translucent and tender. Remove the mushrooms and onions from the pan and set aside.
Reduce the heat to medium and add the butter to the pan. Once the butter is melted, gradually whisk in the flour to create a thick paste. Slowly add the beef broth and heavy cream to the roux, continually whisking as you add.
Once the gravy is smooth and combined, whisk in the Worcestershire sauce and ground mustard then season with salt and pepper to taste.
Stir in the onions and mushrooms then add the chicken back to the pan. Coat the chicken in the sauce and serve over mashed potatoes, pasta, or rice.