1. To prepare the sauce, heat oil in a heavy medium saucepan over medium-low heat. Add shallots, lemongrass and chili and sauté until shallots are translucent, about 3 minutes. Add garlic and ginger and cook until fragrant, about 2 minutes. Add coconut milk and bring to a simmer. Simmer gently for 3 minutes to allow flavors to infuse. Stir in fish sauce, then grate 1 teaspoon of lime zest into sauce and squeeze 1 tablespoon of lime juice into sauce. Return sauce to simmer then remove pan from heat. Cover and set aside to steep for 10 minutes. Strain sauce through fine-meshed sieve and into small saucepan. Cover and keep warm on stove.
2. Cook sugar snap peas in a medium saucepan of boiling salted water until bright green and still crisp-tender, about 1 minute. Drain and transfer sugar snap peas to bowl of ice water to cool completely. Drain again and pat dry with paper towels.
3. To prepare scallops, heat 2 tablespoons of oil in a heavy large sauté pan over high heat. Season scallops with salt and pepper. When oil in pan is just under smoking point, carefully place scallops in pan and cook for 2 minutes on each side, or until brown on outside and just barely translucent in very center. Transfer scallops to plate lined with paper towels to drain any excess oil.
4. Meanwhile, heat remaining 1 tablespoon of oil in another heavy large sauté pan over high heat. Add choy sum and sauté 1 minute. Add baby corn and sauté 1 more minute. Add 2 tablespoons water and sugar snap peas, then sauté until peas are hot and vegetables are still crisp-tender, about 1 minute longer. Season to taste with salt.
5. To serve, place 3 choy sum halves on each plate. Place 1 scallop on top of each choy sum. Place 1 corn and 1 sugar snap pea alongside each scallop and slightly offset on choy sum. Spoon coconut sauce around choy sum, then garnish with coriander sprigs and serve.