Soak the glass noodles: Place the dried glass noodles in boiling water for 1 minute. Rinse under cold water, drain, and set aside.
Prepare the scallops: Gently remove scallops from their shells and rinse under running water to remove any sand. Pat dry and set aside.
Clean the shells: Scrub the half-shells with a clean toothbrush, pat dry, and set aside.
Julienne the kaffir leaf: Separate the leaf from its stalk. Stack the leaves, roll tightly, and slice finely with sharp knife.
Make the dressing: In a bowl, combine lime juice, fish sauce, palm sugar, garlic, chili, coriander, and julienned kaffir leaf. Taste and adjust according to preference — more sour, sweet, spicy, or garlicky.
Prepare lemongrass for steaming: Use the bottom half of each stalk. Remove the tough outer leaves and lightly bruise them with a pestle or the back of a knife to release aroma.
Assemble for steaming: Line a deep dish with the bruised lemongrass and kaffir leaves. Spread the glass noodles on top, then place the scallops over the noodles. Drizzle with 2 tbsp of the dressing. Steam over boiling water for 6–7 minutes, until scallops are just cooked.
Serve: Arrange the glass noodles on each half-shell scallop, then place a scallop on top. Drizzle with the remaining dressing and garnish with sliced chili and coriander. Serve immediately.