Notes: Upo is a long, smooth, light green squash that resembles a short baseball bat. Look for it in Asian markets.
In a food processor or with a knife, mince shrimp with onion. Mix with 2 tablespoons fish sauce and black pepper.
Put shrimp mixture in a 5- to 6-quart pan. Stir over high heat until shrimp mixture is pink and crumbly, about 5 minutes.
Then add 1 1/2 quarts of water to the shrimp mixture and bring to a boil over high heat.
Meanwhile, pare upo with a vegetable peeler (trim zucchini ends, but don't peel). Then cut squash into matchstick-size sticks.
Add squash to boiling shrimp broth. Cover and simmer gently until squash is tender to bite, 5 to 10 minutes.
Ladle soup into bowls, sprinkle with cilantro, and season to taste with more fish sauce and white pepper. Serve hot or cool.