First, remove the mentaiko from the thin sac/membrane. There are many ways to do this. We like to use our fingers to hold one end of the mentaiko and then use chopsticks to press and squeeze all the roe out into a small bowl.You can also use a small knife to cut a slit into the sac/membrane and then scrape out all the roe. Add the kewpie mayonnaise to the bowl with the roe. Mix until evenly combined.
Spread the mentaiko and kewpie mayonnaise mixture on the bread.
Toast at 400F for 3-4 minutes.
Top with a little shredded nori (optional). Eat and enjoy!