Preheat oven to 180°C. Heat 1 tbsp butter in a non stick pan. Add 1 tbsp olive oil and let the butter melt.Drain the sweet corn and rinse in fresh water.
Slice an onion and add to the pan and sauté till soft. Add corn and spinach and mix. Transfer into an baking dish. Sprinkle salt and ½ tsp pepper powder.
Heat the remaining butter in the non stick pan. Spread rice over the vegetables in the baking dish.Add refined flour to the pan and sauté for 1-2 minutes. Add milk gradually, stirring continuously, ensuring there are no lumps.
Cook till the sauce thickens. Add ½ cup cheese and stir.
Cut red and green capsicums (peppers) into small cubes. Sprinkle salt over the capsicums. Add mustard paste to the sauce and mix. Pour the sauce over the rice in the baking dish and spread it all over.
Sprinkle half the capsicums and top it with remaining cheese and sprinkle the remaining capsicums. Drizzle some extra virgin olive oil on top. Bake in the preheated oven for 20-25 minutes. Serve hot.