Cook the udon noodles, drain, rinse and chill in ice water.
I usually use this frozen udon noodle portions that's microwaved for 3.5 minutes, then rinsed and cooled.
Combine the natto, the accompanying sauce and Japanese mustard, an egg, a pitted umeboshi plum, and ground sesame seeds in a serving bowl and start whipping.
Keep whipping the mixture until it's about this foamy. It'll get there in 1 to 2 minutes. Fluffy!
Add the udon noodles, and mix well.
Top with shredded nori seaweed and chopped green onions. Sprinkle ichimi chili pepper powder to add spice and bring the flavors together.
This is a spicy variation with kimchi, salt rubbed sliced cucumbers, and shredded crab stick. I also added sesame seeds and two drizzles of sesame oil.
I discovered that if you mix the natto and egg straight out of the fridge, the mixture becomes even more fluffy and foamy.
This variation has chicken tenders, the white part of a leek, finely chopped, ponzu sauce, plus 2 drizzles of sesame oil. It's refreshing and fluffy.
Story Behind this Recipe
When I was waiting for a pot of rice to cook, I was mixing natto and egg together... when it turned really foamy and voluminous! And it was fluffy-light too! I thought it would go very well with noodles.
-All you need to season these noodles is the sauce that comes with the natto and an umeboshi pickled plum. -Of course, you could use soba noodles or somen noodles instead. -At Step 3, if you whip the egg white to incorporate lots of air, the sauce will become fine, light, and fluffy. -I love to top with a mountain of nori seaweed and green onions. You could also add myoga ginger or shiso leaves.