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