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°F.
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"x9"x2" baking dish.
Pour broth over chicken. Cover loosely with foil and bake for 40 minutes.