Heat oil over medium heat in a nonstick skillet. Add onion and cook, stirring until onions are tender, about 5 minutes. Stir in tomatoes and garlic. Add spinach, cover and cook for about 3 minutes until spinach is soft. Remove from heat. Let mixture cool slightly; stir in Mozzarella and Parmesan.
Heat oven to 375°. Beginning in the center of the thicker end of the breast, insert a small knife horizontally, stopping about 1 inch from opposite end. Open incision to create a 1-inch-wide pocket and pack 1/4 of the filling mixture into each breast.
Season chicken with Italian seasoning blend and place in 13 x 9 x 2-inch baking dish. Pour broth over chicken. Cover loosely with foil and bake for 40 minutes; keep warm until served.
