In a medium bowl, toss together the mango, avocado, red onion, cilantro and lime juice. Cover while preparing halibut.
Preheat oven to 350°F. In small bowl, combine the light brown sugar, paprika, cumin, salt, garlic powder, thyme, black pepper and cayenne pepper. Rub the seasoning into the fish, making sure it's entirely coated.
Heat 1 tablespoon olive oil over medium-high heat in large skillet. Add fish to skillet and cook for 2 minutes per side or until blackened. Transfer to the oven and bake for 5 minutes or until cooked through.
Use wild halibut when possible. It is healthier for you and just tastes better. Frozen halibut works fine too but just make sure it is thawed before cooking.
If you like a really charred blackening, just cook an extra 2 minutes per side on the stovetop. You won't need to throw in the oven to finish.
You know the fish is done when it flakes easily when pressed with a fork.