1. Place the stock in a wok with the lemongrass, lime rind, gingerroot, and chili paste. Bring to a boil, then reduce the heat and summer, covered, for 10-15 minutes.
2. Cut the scallions in half lenthwise, then slice crosswise very thinly. Devein and split shrimp almost in half lengthwise, keeping the tails intact.
3. Pour the stock through a strainer, then return to the wok and bring to a simmer, with bubbles rising at the edges and the surface trembling. Add the scallions and cook for 2-3 minutes. Taste and season with salt, if needed. Stir in a little more chili paste if wished.
4. Add the scallops and shrimp and poach for 1 minute, or until they turn opaque and the shrimp curl.
5. Drop in the cilantro leaves, fill soup in bowls, dividing the shellfish evenly, and garnish with red pepper.