1 large onion, quartered
1 (2-inch-long) piece fresh ginger, peeled
1/2 teaspoon salt
1/2 teaspoon sugar
1/4 cup vegetable oil
1 1/2 teaspoons curry powder (preferably Madras)
1 to 2 fresh serrano chiles, halved lengthwise
1 cup water
1 (14-oz) can unsweetened coconut milk (not low fat)
1 tablespoon fresh lime juice
1 lb large shrimp in shell (21 to 25 per lb)
Accompaniment: cooked basmati rice
Garnish: lime wedges

Pulse onion and ginger in a food processor until finely chopped.
Cook onion mixture with salt and sugar in oil in a 12-inch heavy skillet over moderate heat,
stirring frequently, until onion begins to brown, about 5 minutes.
Stir in curry powder and chiles and cook, stirring frequently, 2 minutes.
Stir in water, coconut milk, and lime juice and simmer, stirring occasionally, until thickened, 5 to 8 minutes.
While sauce simmers, peel shrimp (devein if desired) and season with salt and pepper.
Add shrimp to sauce and simmer, stirring occasionally, until shrimp are just cooked through, about 3 minutes.
Add salt to taste and serve immediately.

Makes 4 servings.