Gather all the ingredients. If you are going to/need to make mentsuyu from scratch, follow the recipe here.
Put wakake seaweed in a small bowl and soak for 15 minutes. After soaking, queeze the water out and set aside.
Thinly slice the scallion.
Peel the cucumber (leave some skin) and thinly slice diagonally. Then cut into julienne strips and set aside.
Peel the daikon skin and grate. Squeeze the water out and set aside.
Bring a large pot of water to boil for udon noodles. My favorite udon is the frozen Sanuki Udon. Blanch the frozen udon noodles in boiling water for 1 minute (no need to defrost). If you use dry noodles, follow the package instructions. Remove udon from boiling water and cool in an ice bath. Wait until it cools completely and drain.
To make the sauce, add ⅓ cup of mentsuyu. My mentsuyu recipe should be diluted about 2-3 times depending on your preference. If you use store bought mentsuyu, read the directions to see if you can use it “straight” (no need to dilute) or if it should be diluted 2 times or 3 times. Add ice cubes to keep mentsuyu cool.
Serve udon on the plate/bowl and add toppings on top. Pour the mentsuyu (sauce) over and enjoy!