Season the river cobbler fillets and vegetables with freshly ground black pepper. Put the vegetables and fish fillets in a large zip-lock food bag. Scatter over the garlic and seal, gently pushing out as much air as possible.
Put the bag on a microwave-safe plate and cook at 800W for 2½-3 minutes, or until the fish flakes easily. Once cooked, set aside to rest for 1 minute.
Meanwhile, in a small bowl, mix all the dressing ingredients together, stirring until the sugar has dissolved. Set aside.
Hold the bag of fish and veg over the sink, then snip off the bottom corner and drain the liquid. Transfer the fish and veg to plates and serve drizzled with the Thai dressing and garnished with the coriander.