Start by making the blackened seasoning. In a small bowl combine the paprika, cayenne pepper, ground black pepper, dried oregano, ground cumin, garlic powder and brown sugar. Mix well. Set the seasoning aside.
Pat dry the grouper fillets using paper towels. This helps to ensure a crispy crust. Brush both sides of the fillets with the melted butter.
Sprinkle a generous amount of the homemade blackened seasoning over both sides of each grouper fillet. Use your fingers to gently press the spice mix into the fillets.
Place a cast-iron skillet over medium-high heat and let it heat up for a few minutes. A cast- iron pan is ideal for achieving that perfect blackened crust.
Add the vegetable oil to the preheated skillet and carefully place the seasoned grouper fillets in the skillet.
Cook the fillets undisturbed for about 3 minutes on each side. The spice blend will create a golden colored flavorful crust and turns to a beautifully blackened fish.
Use a fork to check the doneness of the grouper fillets. The fish should flake easily and the internal temperature on the meat thermometer should be 145 °F (about 62°C)
Once the blackened grouper fillets are cooked, remove them from the skillet.
Serve the lemon wedges and a refreshing mango salsa on the side.