Preparation In a bowl with a fork beat eggs until combined well. Skim any fat from broth and add broth with mirin, soy sauce, and zest to eggs, stirring to combine well. Pour custard through a fine sieve into another bowl. Peel shrimp and devein if desired. Diagonally cut white and pale-green parts of scallion into thin slices and thinly slice mushroom. Divide shrimp, scallion, mushroom, and custard among four 1/2-cup custard cups or ramekins and cover each tightly with foil. Arrange cups or ramekins in a steamer over boiling water and steam custards, covered, over high heat 2 minutes. Reduce heat to moderate and steam custards, covered, until set, about 10 minutes more. Serve custards immediately.
Preparation In a bowl with a fork beat eggs until combined well. Skim any fat from broth and add broth with mirin, soy sauce, and zest to eggs, stirring to combine well. Pour custard through a fine sieve into another bowl. Peel shrimp and devein if desired. Diagonally cut white and pale-green parts of scallion into thin slices and thinly slice mushroom. Divide shrimp, scallion, mushroom, and custard among four 1/2-cup custard cups or ramekins and cover each tightly with foil. Arrange cups or ramekins in a steamer over boiling water and steam custards, covered, over high heat 2 minutes. Reduce heat to moderate and steam custards, covered, until set, about 10 minutes more. Serve custards immediately.