Soak the 100 g glass noodles in cold water until softened, then drain and set aside.
In a bowl, whisk together 200 ml water, 4 tbsp oyster sauce, 2 ½ tbsp light soy sauce, 2 tbsp dark sweet soy sauce, 1 ½ tsp sesame oil, 1 tbsp sugar, the 1 head minced garlic, and the pinch of crushed whole white peppercorns. Stir until the sugar is dissolved.
Add the 4 large fresh prawns (shell-on, deveined) to the sauce and let them marinate for 5 minutes. After 5 minutes, remove the prawns from the bowl and set them aside, keeping the sauce for cooking.
Place a saucepan or clay pot over low heat. Pour the reserved sauce into the pot. Scatter the 4 coriander roots (slightly crushed) and the knob of thinly sliced ginger into the sauce so they sit on the bottom and can infuse the liquid.
Add the drained glass noodles into the pot, spreading them evenly so they sit in the sauce and over the coriander roots and ginger.
Arrange the marinated prawns on top of the noodles. Evenly sprinkle the 1 tbsp ground white pepper over the prawns and noodles.
Cover the pot with a lid and cook over low heat until the prawns turn pink and are just cooked through and the noodles have absorbed most of the sauce, about 6–8 minutes.
Remove from heat, transfer to a serving dish, and garnish with the chopped spring onions (3 stalks) and chopped Chinese celery (3 stalks) before serving.