Cut mochi into bite-sized pieces and microwave for 2 minutes at 600W.

Cut potatoes into bite-sized pieces, soak in water, drain, and microwave for 4 minutes at 600W with a pinch of salt.

Thinly slice the onion and extract the contents of the mentaiko.

Heat butter in a pan, add onion, and sauté over medium-low heat until softened.

Add potatoes to the pan, sauté briefly, then add butter and stir until melted. Add flour and cook until no longer powdery.

Gradually add milk in two portions, stirring well each time.

Once slightly thickened, add salt and mentaiko, mixing well to create the white sauce.