Heat the oil in a large casserole dish . Add the onions and cook for 6 to 8 minutes, stirring occasionally, until they are softened. Add the tomatoes, spinach, rice, broth and pepper. Bring to a boil, stirring as needed.
Bake in a preheated oven until the rice is tender and most of the liquid is absorbed, about 1 hour.
Notes
For Serving:
3/4 cup feta cheese, crumbled
10 black olives, pitted & halved