Shrimp and Avocado in Tamarind Sauce

1 large shallot , thinly sliced and separated into rings

1/4 cup tamarind pulp (from a pliable block)

1/2 cup boiling-hot water

2 tablespoons sugar

1 tablespoon soy sauce

1 tablespoon Asian fish sauce

3 tablespoons fresh lime juice

2 firm-ripe California avocado s

1 tablespoon minced peeled fresh ginger

1 garlic clove, minced

2 (1 1/2- to 2-inch-long) fresh Thai chiles or 1 serrano, stemmed and minced (including seeds)

1/4 teaspoon salt

1 lb large shrimp in shell (21 to 25 per lb), peeled, leaving tail and first segment of shell intact, and deveined

1/3 cup roasted salted peanut s, chopped

Accompaniment: jasmine rice

Garnish: fresh cilantro sprigs