Select three small ceramic bowls that can be steamed. Find a large pot or pan that can comfortably sit all three at once, fill the pan with enough water to go up the sides of the bowl by a few inches. Remove the bowls, turn on the heat and bring the water to a gentle simmer.
Whisk together the egg and dashi.
Put a small handful of udon noodles that have been cut into shorter lengths into the bottom of each bowl, top with desired add ins.
Pour the custard into the bowls, each one should get enough to mostly cover the noodles, the toppings to not have to be fully submerged.
Cover each bowl with a pieces of plastic wrap and place into the simmering water. Turn down the heat to medium low and cover the pan with a lid or tin foil.
Simmer for 12 minutes or until a toothpick inserted into the egg comes out clean. These are best served warm or room temperature.