Cook the somen according to instructions on the package. Drain immediately and wash with your hands in cold water to cool completely and get rid of starch. Keep in cold water until ready to eat.
Make the dipping sauce by combining the dashi stock, soy sauce, sake, and mirin in a small saucepan over a low heat. Taste and adjust seasonings: for a saltier sauce, add more soy sauce, for a sweeter one, more mirin. Chill before using; if made in advance, refrigerate until ready to use.
To serve, place cubed ice in a large glass bowl or individual ones. Drain the somen and place it on the ice. Arrange the garnishes on top. Pour the dipping sauce into four small glass bowls and arrange mounds of condiments on a large dish or several small ones so that guests can mix whichever condiments they like into their sauce and use it as a dip for mouthfuls of somen, vegetables, and fruit