Make small slit in each shrimp; pull tail through.
Wilt spinach in hot water, drain.
In each of eight 5 ounce custard cups (or Chawan Mushi cups), place shrimp, spinach, mushrooms, and water chestnuts.
Combine eggs, chicken broth, and salt; pour into cups; cover with foil.
Set cups on rack in Dutch oven; pour hot water around cups 1 inch deep; cover to steam.
Over medium heat, bring water slowly to simmering; reduce heat and cook 7 minutes or till knife inserted off center comes out clean.
Top each custard with 1/4 teaspoon soy sauce and a twist of lemon peel.