Peel chayote and cut in 1/8-1/4" dice. Peel shrimp; combine shells in a pot with lemon rind, optional lemon grass tops, allspice berries, garlic, 2 tablespoons oyster sauce, and water. Simmer, covered, for 15-20 minutes.
Strain shrimp broth, discarding solids. Return to the pot with the chayote; simmer until not quite tender, 5-6 miuntes.
Devein and halve shrimp lengthwise; add to soup with mushrooms and 1 teaspoon chili paste. Bring soup to a boil, stirring until shrimps turn pink. Slowly add lemon juice, chili paste, and oyster sauce to taste. Stir in cilantro and serve.
Source: Uncommon Fruits and Vegetables - A Commonsense Guide, Elizabeth Schneider