How to Make It
Wrap a 10-inch cast-iron skillet in aluminum foil, covering entire base of skillet. Set aside.
Pat chicken dry with paper towels, and place on a work surface. Carefully run a sharp knife along the bones, and pull the meat away from bones. Discard bones. (Do not remove skin on breast meat.) Season both sides of each breast evenly with salt and pepper. Heat oil in a 12-inch nonstick skillet over medium-high. Place chicken, skin side down, in skillet. Place prepared cast-iron skillet, bottom side down, on chicken. Cook until skin side is golden brown and crispy, 8 to 10 minutes, checking after 8 minutes. Turn chicken breasts, and place cast-iron skillet, bottom side down, on chicken. Cook until done, 5 to 6 minutes. Remove from skillet, reserving drippings in skillet, and cover chicken to keep warm.
Add onion and garlic to drippings in nonstick skillet. Cook over medium, stirring often, until onions are softened, 5 to 6 minutes. Add marinara sauce, and cook, stirring often, until sauce thickens slightly, 4 to 5 minutes. Carefully nestle chicken breasts, skin side up, in sauce in a single layer. (Make sure chicken skin stays out of the sauce to ensure skin stays crispy.) Arrange mozzarella slices over sauce. (Mozzarella may slightly overlap chicken breasts.) Cover and cook until cheese melts, 3 to 4 minutes. Sprinkle with basil. Serve chicken, cheese, and sauce over pasta.