1/3 cup panko (Japanese breadcrumbs)
2 tablespoons grated Parmesan cheese $
2 teaspoons unsalted butter, melted $
1 tablespoon olive oil, divided $
4 (6-ounce) skinless, boneless chicken breast halves $
1/2 teaspoon kosher salt, divided $
1/2 teaspoon freshly ground black pepper, divided $
1 cup chopped onion $
2 teaspoons minced garlic
1 cup chopped tomato $
3/4 cup lower-sodium marinara sauce (such as Dell'Amore)
1/4 cup unsalted chicken stock (such as Swanson) $
1 tablespoon chopped fresh oregano, divided
2 ounces fresh mozzarella cheese, torn into small pieces (about 1/2 cup)