Cut the mentaiko lengthwise and scoop out the contents with a spoon. Mix it with mayonnaise in a bowl.

In a separate bowl, beat the eggs and mix in milk, salt, and pepper.

Heat a frying pan over medium heat with a thin layer of salad oil. Add unsalted butter and let it melt halfway, then pour in the egg mixture. Stir gently with chopsticks until the eggs are semi-cooked.