Place a rack in the center of the oven and preheat to 375°F (190°C).
Pat the chicken breasts dry and season both sides with the kosher salt and black pepper.
Heat 1 tablespoon of the olive oil in a large ovenproof skillet over medium-high heat. When hot, add the chicken and sear 2–3 minutes per side, until golden brown; the chicken does not need to be cooked through. Transfer chicken to a plate.
Reduce heat to medium and add the remaining 1 tablespoon olive oil to the skillet. Add the chopped onion and sauté until tender, about 5 minutes.
Add the minced garlic and Italian seasoning to the skillet, then pour in the pizza sauce. Stir and let the sauce simmer for 5 minutes.
Return the seared chicken to the skillet, nestling pieces into the sauce. Sprinkle the shredded mozzarella evenly over the chicken, then arrange the pepperoni slices on top.
Transfer the skillet to the preheated oven and bake 20–25 minutes, or until an instant-read thermometer inserted into the thickest part of the chicken reads 165°F (74°C).
Remove from the oven and let rest a few minutes before serving.
