Make a 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), please 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 (that's the key) to simmering; reduce heat and cook 7 minutes or until knife inserted off center comes out clean. Top each custard with 1/2 t. soy sauce and a twist of lemon peel.