Rinse the rice under cold water until the water runs clear. In a large pot, bring the broth to a boil. Add the rice, reduce the heat to low, cover, and simmer for about 15-20 minutes or until the rice is tender and the liquid is absorbed. Remove from heat and let it sit covered for 5 minutes.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for about 5 minutes until translucent. Stir in the minced garlic and cook for another minute.

Add the chopped spinach, parsley, oregano, salt, and pepper to the skillet. Cook for about 2-3 minutes until the spinach is wilted.

Fluff the cooked rice with a fork and add it to the skillet with the herb mixture. Stir well to combine.

Preheat the oven to 400°F. Place the fish fillets on top of the rice mixture in the skillet. Season the fish with a little salt and pepper, and top with lemon slices.

Transfer the skillet to the oven and bake for 15-20 minutes, or until the fish is cooked through and flakes easily with a fork. If using, sprinkle the grated Parmesan cheese on top before serving.