Soak soybeans for at least 6 hours, then boil them in water.
Blend the boiled soybeans to make soy milk.
Boil chicken breast and shred it along the grain.
Slice cucumber and onion into thin strips.
Boil thin wheat noodles for cold soup and rinse them in cold water.
Pour cold soy broth into a bowl and add the noodles.
Top with shredded chicken breast, cucumber, and onion.
Season with salt, vinegar, and mustard.