Prepare Toppings: Chop scallions. Soak the wakame seaweeds for 5 minutes and drain completely.
Cook Udon Noodles: Bring a medium saucepan of water to a boil, add the udon noodles, and cook according to the package instructions. Stir the noodles with chopsticks to separate them as they cook.
Drain the Noodles: Drain using a strainer and quickly rinse under running cold water to remove the extra starch.
Udon Soup: Put mentsuyu and water in the same saucepan and warm it up.
Serve: Serve the drained noodles in a bowl, pour the soup, and place the scallions and wakame on top.
Notes
If the udon soup is too salty, dilute it with water. If it's too light, add more mentsuyu.
Udon noodle cooking time - a couple of minutes for frozen ones. 10 minutes or more for fresh and dried ones.
Try various toppings from the "Topping variations" section.
If you use store-bought mentsuyu, follow the instruction on the package. How much water to add depends on the product.