Light a grill and heat until hot. Combine horseradish, zests and cilantro, mix well and season with salt and pepper. Season trout fillet with Creole spice. Lay trout fillet on a well-oiled cedar plank and top with horseradish mixture, packing it firmly around fish. Place plank on grill, cover immediately. Grill will begin to smoke a lot -- don't worry, it's the wood smoldering. Check for doneness after 5 minutes; be careful not to overcook. Meanwhile, whisk together remaining ingredients. To serve, carefully remove fish from plank with a spatula and pour sauce over.
Yield: 1 serving