Preheat the oven to 400°F (200°C). Pour the jar of pizza sauce into a small saucepan and simmer over low heat, stirring occasionally, until it is reduced to 1 cup (about 20 minutes). Set the reduced sauce aside.

Trim any excess fat from the chicken breasts. Cut each breast crosswise into two same-size pieces (you will have 8 pieces total).

Place one chicken piece at a time into a heavy plastic bag and pound with a meat mallet or heavy pan until as thin as you can get it without shredding the meat. Repeat with remaining pieces. Pat the pieces dry with paper towels if they are wet.

In a small bowl, mix 1 tsp Greek oregano and 1 tsp garlic powder. Evenly sprinkle this seasoning mixture on both sides of each chicken piece.

Heat 1 T olive oil in your largest nonstick frying pan over medium-high heat. Working in a single layer (do batches if necessary), add the chicken pieces and cook 1–2 minutes per side just to brown them—do not cook through. Transfer the browned pieces to a plate as you finish each batch.

Choose the smallest glass casserole dish that will fit all the browned chicken pieces in a single layer. Arrange the browned chicken pieces in the dish.

Slice the 6 oz piece of mozzarella about 1/4 inch thick and cut the slices as needed so you have enough to mostly cover the top of each chicken piece.

Spread the reduced pizza sauce evenly over the top of each chicken piece, using all of the sauce.

Top each chicken piece with the mozzarella slices and then distribute the 2 oz sliced pepperoni over the tops so they mostly cover the cheese.

Bake uncovered at 400°F (200°C) for 25–30 minutes, until the cheese is melted and beginning to brown and the pepperoni is slightly crisp. (Check that chicken is cooked through — an internal temperature of 165°F/74°C is a good indicator.)

Let the dish rest a few minutes, then serve hot.