Fen si bao (粉丝煲, glass noodle casserole) is a Cantonese and northern Chinese preparation — dried glass noodles (fen si, 粉丝, made from mung bean starch) simmered in a clay pot with a savoury broth of oyster sauce, soy sauce, and aromatics, often with additions of prawn, minced pork, tofu, or vegetables. The glass noodles absorb the braising liquid completely, becoming translucent, slippery, and deeply flavoured. The clay pot retains heat at the table, continuing to simmer gently through the meal.
The key technique: Glass noodles must be pre-soaked in cold water for 15-20 minutes before cooking — they will expand significantly and become pliable. Do not soak in boiling water — they will overcook in the casserole. The casserole: In a clay pot, heat oil and fry aromatics (garlic, ginger, scallion). Add protein (minced pork or prawns) and cook until just done. Add the pre-soaked, drained glass noodles. Add enough braising liquid (a mixture of chicken stock, oyster sauce, light soy sauce, and dark soy sauce for colour) to almost cover the noodles. Bring to a boil in the pot. Cover and reduce to a simmer for 5-8 minutes — the noodles should absorb most of the liquid. Finishing: Drizzle sesame oil over. Scatter sliced scallion. Serve in the clay pot.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)