In a small saute pan on medium heat, add 1 tablespoon oil and saute lemongrass and scallion whites until soft, about 4 minutes. Season and set aside. In a bowl, whisk together juice, coriander, scallion greens and remaining oil. Add lemongrass and check for seasoning. In another bowl, mix crab and fennel. Add 2/3 of the vinaigrette and toss to coat. Check for seasoning.