Wash tilapia, cut it with a knife, smear it with salt and pepper, and marinate it with a little soy sauce for five minutes. Shred onion and ginger, and cut coriander into sections. Prepare enough boiled water. Put cold oil in a hot pan, fry the fish until it is solidified, turn over and fry until it turns yellow. Pour in enough boiling water, simmer over high fire, and cook for 15 minutes without any seasoning, so as to get the milk soup. Ten minutes later, the color of the soup has begun to turn white. Add the tofu pieces and cook for another five minutes to taste. Season with salt and pepper and take out.