This soup started out as a way to turn extra egg whites (leftover from making mint chip ice cream) into dinner. It's very quick, very easy, very savory, and very light. In fact, it was a bit too light for a light dinner for three, and would be even better bulked up with wontons, sturdy greens, ramen or udon noodles, etc—try adding those between the mushrooms and the egg.
drizzle peanut oil
1 thumb-size chunk of ginger, peeled and sliced
1 bunch scallions (6-8), half chopped in 2" lengths and half thinly sliced
1/2 t red pepper flakes
dozen shiitake mushrooms, stems reserved and tops quartered
6 c hot stock (2 not-chicken stock cubes)
1 t soy sauce, or to taste
6 oz tofu, diced
1 T cornstarch
3 egg whites, lightly beaten
Heat the oil, ginger slices, long scallion parts, pepper flakes, and mushroom stems in a pot while you boil water for the stock. Add the stock to the pot and simmer for 15 minutes until the the broth is flavored to taste (add soy sauce now as needed). Remove and discard the ginger, scallions, and mushroom stems (scooping or straining works).
Add the tofu and mushroom tops to the broth and simmer until the mushrooms are cooked through. Ladle a bit of broth into a bowl and dissolve the cornstarch in it, then stir it back into the pot to thicken the broth a bit. (This should help the chunky bits stay suspended in the broth instead of just floating at the surface.)
With the soup at a very gentle simmer, slowly drizzle the egg whites in a thin stream around the surface of the pot—the egg should cook instantly into ribbons. Sprinkle in the rest of the scallions and gently stir. Simmer for another couple minutes to set the egg completely before serving.