Preheat the Oven: Begin by preheating your oven to 350°F (175°C).
Prepare the Spinach Mixture: In a large mixing bowl, combine the thawed and drained spinach, ricotta cheese, mozzarella cheese, Parmesan cheese, and beaten egg. Season with garlic powder, onion powder, crushed red pepper flakes, salt, and pepper. Mix well to ensure all ingredients are evenly combined.
Add Quinoa or Rice: Stir in the cooked quinoa or rice until fully incorporated into the cheesy spinach mixture.
Prepare the Casserole Dish: Grease a 9x13 inch casserole dish with cooking spray or olive oil. Spread half of the herb seasoned stuffing mix evenly on the bottom of the dish.
Layer the Mixture: Spoon the cheesy spinach filling over the stuffing layer, spreading it out evenly.
Top with Remaining Stuffing: Sprinkle the remaining herb seasoned stuffing mix on top of the spinach filling layer.
Pour in the Broth: Carefully pour the vegetable broth over the casserole, allowing it to soak through the stuffing without disturbing the layers.
Bake: Cover the dish with aluminum foil and bake in the preheated oven for about 25 minutes. Then, remove the foil and bake for an additional 15 minutes or until the top is golden and crispy.
Garnish and Serve: Remove the casserole from the oven and let it cool for a few minutes. (Optional) Garnish with fresh parsley before serving.