Preheat oven to 350 degrees F (175 degrees C). Prepare a 9x13-inch baking dish with cooking spray.
Mix cooked brown rice, black beans, diced tomatoes with green chile peppers, ricotta cheese, mozzarella cheese, Parmesan cheese, parsley, and black pepper in prepared dish.
Bake in preheated oven until cheese is bubbling, about 30 minutes. Garnish with tomato slice.
