Prepare the mentaiko by slicing it open and extracting the contents. Reserve a small portion for topping.

In a microwave-safe bowl, add butter and cover loosely with plastic wrap. Microwave at 600W for 30 seconds to melt the butter. Add flour and mix well to avoid lumps.

Gradually add milk while mixing. Microwave uncovered at 600W for 2 minutes, then stir. Microwave again for another 2 minutes and stir until thickened.

Add mentaiko and pepper to the sauce and mix well.

Prepare the butter rice by mixing cooked rice, butter, and soy sauce in a bowl until the butter melts and is evenly distributed.

Divide the butter rice into heatproof containers. Top with mentaiko white sauce and pizza cheese. Bake in a toaster oven for about 5 minutes until golden brown.