Cut the onion into wedges, and warm the cutlet in the microwave or toaster, then cut it into bite-sized pieces.

In a frying pan, bring the mentsuyu, water, and onion to a boil over medium-low heat.

Once the onion becomes translucent, place the cutlet on top and pour in the beaten eggs from 2 eggs. Cover with a lid for about 1 minute.

Pour in the remaining beaten egg, turn off the heat, cover with the lid, and let it sit for about 20 seconds until the egg is softly set.