A version of Vietnamese Hot and Sour Shrimp Soup (Canh tôm nấu chua) is made throughout Vietnamese Cuisine, though it probably originated in the south of the country, where tropical fruit and tamarind are abundant. The pineapple gives the soup sweetness while the tamarind lends tartness (vinegar or lime juice can also be used as a souring agent). Bac ha is the stem of the taro plant. It’s a celery-like vegetable that stays crisp even when simmered in soup. It’s fun to look for it in Asian grocery stores, but if you can’t find it, don’t worry - Vietnamese Hot and Sour Shrimp Soup (Canh tôm nấu chua) will be just fine without it, though I sometimes add some shredded iceberg lettuce in its place.
In a large pot, combine the stock, tamarind pulp, and chiles and bring to a boil over high heat. Lower the heat to a steady simmer and simmer for 15 minutes, stirring occasionally with a whisk to break up the tamarind pulp. Remove from the heat, strain through a fine-mesh sieve, and discard the solids. Return the liquid to the pot.
Stir in the fish sauce, lime juice, and salt and bring to a simmer over medium heat. Add the shrimp and pineapple and cook for about 30 seconds, until the shrimp just turn pink.
Divide the bean sprouts and bac ha among warmed soup bowls. Ladle the hot soup into the bowls and garnish with the cilantro and toasted garlic, dividing them evenly. Serve immediately.