1/2 cup chopped scallion
4 garlic cloves, crushed
3 fresh thyme sprigs
3 fresh Scotch bonnet or habanero chiles, halved and seeded
2 tablespoons salt
1/2 teaspoon black pepper
10 whole allspice
1 lb large shrimp in shell (21 to 25 per lb)
Combine all ingredients except shrimp in a 4-quart heavy pot and bring to a boil, then reduce heat and simmer, covered, 20 minutes.
Stir in shrimp, making sure they are just covered by the liquid, and remove pot from heat. Cool everything to room temperature, uncovered, about 1 hour. Transfer shrimp with a slotted spoon to a plate or bowl and drizzle some of cooking liquid on top.