Put the potatoes in the water with a tablespoon of salt.
Bring to a boil, and cook for 30 minutes or until the potatoes are soft.
Add the onion and tomatoes.
Slowly, stir in the egg whites.
Separately, mix the egg yolks with the milk.
Add the pan tostado.
Take the caldo off the heat and, while still boiling, add the yolk/milk mixture.
Stir and serve immediately, sprinkled with cilantro.