Preheat your oven to 180 C (350 F) and grease with butter a 7x11 baking dish.
In a large mixing bowl, mix together the rice, butter, spinach, Cheddar cheese, 3/4 cup of Parmesan cheese, spring onion, garlic, milk, and eggs, season with salt and pepper as desired.
Pour into the baking dish and sprinkle with the remaining Parmesan cheese.
Bake for 25 minutes, uncovered, then optionally top with Mozzarella cheese and bake for 5 more minutes.
Let casserole rest for 5 minutes (there is a lot of hot cheese in there), then serve as a light main dish or as a side dish with say, chicken or fish. Enjoy.