2 ¼ cups dry Marsala
4 teaspoons cornstarch
2 T water
8 White Mushrooms - sliced
4 (6- to 8-ounce) boneless, skinless chicken breasts, trimmed
Kosher salt and pepper
2 cups beef broth
¾ cup all-purpose flour
¼ cup plus 1 teaspoon vegetable oil
3 ounces pancetta, cut into 1/2-inch pieces
1 pound cremini mushrooms, trimmed and sliced thin
1 shallot, minced
1 tablespoon tomato paste
1 garlic clove, minced
2 teaspoons lemon juice
1 teaspoon minced fresh oregano
3 tablespoons unsalted butter, cut into 6 pieces
2 teaspoons minced fresh parsley
1. Cut each chicken breast in half crosswise, then cut thick half in half again horizontally, creating 3 cutlets of about same thickness. Place cutlets between sheets of plastic wrap and pound gently to even 1/2-inch thickness. Place cutlets in bowl and toss with 2 teaspoons salt and 1/2 teaspoon pepper. Set aside for 15 minutes.
3. Pour Marsala into saucepan, add broth, and boil over high heat. Lower heat to medium-high and simmer until reduced to 1 1/2 cups, 10 to 12 minutes. To thicken whisk corn starch and water together in a small bowl and stir into Marsala mixture to thicken. Set aside.
4. Spread flour in shallow dish. Working with 1 cutlet at a time, dredge cutlets in flour, shaking gently to remove excess. Place on wire rack set in rimmed baking sheet. Heat 2 tablespoons oil in 12-inch skillet over medium-high heat until smoking. Place 6 cutlets in skillet and lower heat to medium. Cook until golden brown on 1 side, 2 to 3 minutes. Flip and cook until golden brown on second side, 2 to 3 minutes. Return cutlets to wire rack. Repeat with 2 tablespoons oil and remaining 6 cutlets.
5. Return now-empty skillet to medium-low heat and add pancetta. Cook, stirring occasionally, scraping pan bottom to loosen browned bits, until pancetta is brown and crisp, about 4 minutes. Add cremini and white mushrooms and increase heat to medium-high. Cook, stirring occasionally and scraping pan bottom, until liquid released by mushrooms evaporates and mushrooms begin to brown, about 8 minutes. Using slotted spoon, transfer mushrooms and pancetta to bowl. Add remaining 1 teaspoon oil and shallot to pan and cook until softened, 1 minute. Add tomato paste and garlic and cook until fragrant, 30 seconds. Add reduced Marsala mixture, remaining 1/4 cup Marsala, lemon juice, and oregano and bring to simmer.
6. Add cutlets to sauce and simmer for 3 minutes, flipping halfway through simmering. Transfer cutlets to platter. Off heat, whisk in butter. Stir in parsley and mushroom mixture. Season with salt and pepper to taste. Spoon sauce over chicken and serve.