Combine grated zucchini, thawed spinach (excess water squeezed out), finely sliced spring onion, rice and grated cheeses in a large bowl and toss to coat.
Whisk together the eggs, milk and pepper and stir into the rice mixture.
Spread rice mixture into a deep 20cm x 30cm baking paper lined dish, cover with foil and bake at 180°C for 30 minutes.
Remove foil and cook for a further 30 minutes until eggs have set and rice is cooked.
Cool to room temperature before serving or refrigerate until required.
To serve, cut into 12 squares and serve warm or cold.