For the relish: Combine the tomatillo, avocado, serrano, fennel and onions in a medium bowl. Drizzle with the canola oil, lime juice, vinegar and honey. Season with salt and pepper, and fold in the cilantro.
For the crispy skin fish: Heat a saute pan over medium-high heat until very hot. Add some canola oil and heat until hot and smoking.
Meanwhile, sprinkle the fish on both sides with salt and pepper. Lay the fish in the hot pan, skin-side down, pressing down with your fish spatula. Add the butter to the pan and let melt. When the edges of the fish look golden brown and crispy, after 3 minutes, flip the fish. Reduce the heat and cook another 2 minutes. Using a spoon, baste the crispy skin with some of the butter.
Remove the fish to a plate. Serve with the relish. Finish with some extra-virgin olive oil.