To make the soup, bring the dashi to a boil over medium high heat. Once boiling, add mirin, soy sauce and salt. Bring to a boil again and then turn off and set aside.
Then, boil 3 cups of water and add the udon noodles. Make sure to separate them out while they cook. Once they are separated, remove them from heat and place in a bowl of ice water to prevent them from continuing to cook.
Place your broth in a bowl and ladle the noodles in slowly. Top your soup with sliced cucumbers, bonito flakes, sesame seeds, seaweed and green onion or anything else you want to add!