Slice the onion thinly.

Cut open the mentaiko and extract the contents.

Boil water in a pot, add salt, and cook the spaghetti 1 minute less than the package instructions.

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

Add flour to the pan and stir until no raw flour remains. Gradually add milk while stirring until a light creamy consistency forms. Season with salt, pepper, and mentaiko.