Thinly slice the onion and remove the mentaiko flesh from its casing.

In a bowl, beat the egg and mix with mentaiko and grated cheese.

Heat butter in a frying pan and sauté the onion over medium heat until softened.

Add milk, water, salt, and consomme powder to the pan and bring to a boil. Add spaghetti, breaking it in half, and stir.

Cover the pan and cook the spaghetti over low heat for 2 minutes longer than the package instructions, stirring occasionally.