Cook udon noodles: Boil plenty of water (not in the ingredients list) in a medium saucepan, add udon noodles, and cook according to the package instructions. Stir the noodles with chopsticks to separate them while cooking.
Drain the noodles: Drain using a strainer and quickly rinse under running cold water to remove the extra starch.
Curry soup: Put leftover curry, 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 on top.
Notes
If the curry soup is too salty, dilute it with water. If it's too light, add more mentsuyu or leftover curry.
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.