• Preheat the oven to 200 degrees F.
• Adjust oven rack to lower-middle position, place large heatproof dinner plate on oven rack.
• Heat 12-inch heavy-bottomed skillet over medium-high heat until very hot (you can hold your hand 2 inches above pan surface for 3 to 4 seconds), about 3 minutes.

Prepare the chicken:

• Pat chicken breasts dry.
• Place flour in shallow baking dish or pie plate.
• Season both sides of chicken cutlets with salt and pepper; working one piece at a time, coat both sides with flour. Lift breast from tapered end and shake to remove excess flour; set aside.

Brown the cutlets:

• Add oil to hot skillet and heat until shimmering.
• Place floured cutlets in single layer in skillet and cook until golden brown, about 3 minutes.
• Using tongs, flip cutlets and cook on second side until golden brown and meat feels firm when pressed with finger, about 3 minutes longer.
• Transfer chicken to heated plate and return plate to oven.

Brown the pancetta:

• Return skillet to low heat and add pancetta; sauté, stirring occasionally and scraping pan bottom to loosen browned bits until pancetta is brown and crisp, about 4 minutes.
• With slotted spoon, transfer pancetta to paper towel--lined plate.

Sautée the mushrooms and aromatics:

• Add mushrooms and increase heat to medium-high; sauté, stirring occasionally and scraping pan bottom, until liquid released by mushrooms evaporates and mushrooms begin to brown, about 8 minutes.
• Add garlic, tomato paste, and cooked pancetta; sauté while stirring until tomato paste begins to brown, about 1 minute.

Make the sauce and serve:

• Off heat, add the Marsala.
• Return pan to high heat and simmer vigorously, scraping browned bits from pan bottom, until sauce is slightly syrupy and reduced to about 1 1/4 cups, about 5 minutes.
• Off heat, add lemon juice and any accumulated juices from the chicken.
• Whisk in butter 1 tablespoon at a time.
• Season to taste with salt and pepper, and stir in parsley.
• Pour sauce over chicken and serve immediately.