Prepare the Onion Rings

1. In a large, deep sauté pan or fryer, preheat enough vegetable oil to cover the onions. Heat oil to approximately 350°F (175°C).
2. In a medium bowl, combine 1 cup of flour and the blackening spice. Toss the sliced onion rings in the flour mixture, shaking off any excess.
3. Fry the onion rings in the preheated oil for 4-5 minutes, stirring occasionally, until dark brown. Remove from oil and place on paper towels to drain.

Create the Onion-Panko Crust

1. Once the fried onion rings have cooled, finely chop them using a kitchen knife or food processor. The onion pieces should be about the size of panko breadcrumbs.
2. Combine the chopped onion rings with the 1/2 cup of panko breadcrumbs in a bowl and mix well.

Dredge the Yellowtail

1. Dredge the yellowtail fillets first in the remaining 1/2 cup of flour, then through the beaten eggs, and finally press them into the onion-panko mixture, ensuring a good coating on all sides.

Cook the Yellowtail

1. In a large sauté pan, add enough vegetable oil to coat the bottom of the pan and heat over medium-high heat.
2. Cook the breaded yellowtail fillets for 3-4 minutes per side, or until the onion crust is golden brown, and the fish is cooked through and opaque.

Make the Key Lime Butter

1. In a small saucepan, melt the stick of butter over medium heat. Add the 1/4 cup of white wine and the juice of 6 Key limes.
2. Stir well and remove from heat. Let the sauce rest at room temperature.

Prepare the Mango Salsa

1. In a medium bowl, combine the diced mango, green bell pepper, tomato, and red onion. Add the juice of 4 Key limes and season with salt and pepper to taste.
2. Mix in fresh cilantro to taste. Set aside.

Serve

1. Top the fried yellowtail fillets with a drizzle of Key lime butter and a generous spoonful of mango salsa.
2. Serve with your choice of sides like a light salad or steamed vegetables.