Place the ginger, lemongrass, and garlic in an oiled pan over medium-high heat. Cook until it becomes fragrant, about 5 minutes. Add the seafood stock.
Bring to a simmer and heat for 5 minutes. Remove the ginger and lemongrass. Reduce the heat, add the daikon and carrots, and allow it to cook for 30 minutes.
While the soup simmers, preheat the oven to 400°F. Season the salmon with garlic powder, ginger powder, salt, and pepper. Prepare a baking sheet with aluminum and nonstick spray. Place salmon on the baking sheet and bake for 18 minutes. Set aside until the soup is ready.
Add the coconut milk to the soup and stir well.
Add the bok choy and mushrooms. Cook until the the bok choy and mushrooms are cooked through. Finally, add the fish sauce and lime juice. Season with salt, pepper and cayenne to your liking.
To serve, place some udon in a bowl. Pour the soup and top with two piece of salmon.
