In a large, deep saute pan or fryer, preheat enough vegetable oil to cover onions. Heat oil to approximately 350 degrees Fahrenheit. In a medium bowl place 1 cup flour and blackening spice. Toss sliced onions in flour mixture and lightly shake off any excess flour. Place dusted onion rings into hot oil, and fry for 45 minutes, stirring onions occasionally, until onions are dark brown. Remove onions from the oil, and place on paper towels to remove any excess oil. Allow onion rings to cool to room temperature.
Then using a kitchen knife or a food processor, chop onion rings until they are about the size of the panko bread crumbs. Place chopped onion rings and panko bread crumbs into a medium bowl, and mix well. Dredge lobster tails through remaining flour, then through the beaten eggs, and then press yellowtail filets into the onion ring and panko bread crumb mixture.
Place a large saute pan on the stove and add enough oil to just coat the bottom of the pan. Place battered filets into the saute pan and saute over medium high heat for 34 minutes per side, or until onion crust is lightly browned, and fish is white throughout. Top with Key lime butter and mango salsa....and enjoy!
For the Key Lime Butter (prepare in advance): Melt butter over medium heat, add dry white wine, and key lime juice. Remove from heat and let rest at room temperature.
For the Mango Salsa (prepare in advance): In a bowl, add diced mango, green peppers, diced tomatoes, red onions, Key lime juice, and salt and pepper, and mix well.