Prepare the egg. Bring a small pot of water to a boil. Once boiling, turn off the heat. Gently add the egg. Put a lid on the pot and remove from heat. Let sit for 6 minutes.
Prepare everything else while the egg is cooking. Start with the udon:Bring a small pot of water to a boil. Boil the frozen udon noodles for 1 minute, and drain. Rinse under running water (for 10-15 seconds) and drain again. Place the noodles in a serving bowl. Prepare the mentaiko: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. Top the udon with the mentaiko, chopped green onions, and shredded nori.
Pour the mentsuyu into a small cup/sauce dish. Microwave for 10 seconds. Set aside.
Once the soft-boiled egg is done, crack it open directly into the bowl with the udon and all the toppings.
Add shichimi togarashi (optional) if using.
The dish is ready! Serve the udon bowl with the mentsuyu on the side. Pour on as much of mentsuyu as you like. Mix, eat, and enjoy!