This is a really light and flavorful soup. I recommend making your own stock, but store-bought is fine, too.
Serves 4-6
Ingredients:
2 lemongrass stalks, 1-2 hard outer layers discarded
2 large shallots, peeled and thinly sliced
2 garlic cloves, peeled and thinly sliced
2 tbsp vegetable oil
2 quarts chicken stock
1 14oz can diced tomatoes, drained but reserving juice
2oz tamarind paste
3 tbsp fish sauce
2 Serrano chiles, thinly sliced
A little lime zest