Shrimp in Tamarind Sauce

Tamarind is a large seed pod grown and used in the cuisine of almost every equatorial country. Its pulp is processed into a dark brown paste, which is used as you might tomato paste, concentrated stock, or, for that matter, ketchup—as a simple flavor enhancer that completely changes the nature of the sauce into which it is stirred. You can make your own paste (page 585) or buy the paste now sold in bottles at many Asian stores. (There is also a kind of instant tamarind powder, which is not very good.) The bottled paste is concentrated to increase its strength, so if you use homemade tamarind paste, double the quantity (or add to taste). As for this recipe, you can use the same process to make almost anything in a tamarind sauce, but shrimp, because it requires minimal cooking time, is by far the easiest. I like to brown the shrimp first for a little more flavor, but you could actually start this recipe with the second step and add the shrimp after the onion. The dish is best when quite sour and not—as happens at too many restaurants—a sticky combination of sweet and sour.