Preheat oven to 375°.
Remove large stems from spinach. Tear spinach into 1-inch pieces; place in a colander. Rinse spinach under cold water; drain. Set aside.
Place a large Dutch oven coated with cooking spray over medium heat until hot. Add onion; sauté 3 minutes. Add spinach; cover and cook 2 minutes or until spinach wilts. Add cream cheese, oregano, salt, and pepper. Uncover and cook an additional minute or until cream cheese melts. Spoon spinach mixture into a 1-quart gratin dish or shallow casserole coated with cooking spray. Arrange tomato slices in a single layer on top of spinach,and sprinkle with breadcrumbs and Parmesan cheese. Bake at 375° for 30 minutes or until golden brown.