Preheat 2 small cast-iron skillets over high heat until hot, about 5 minutes. Meanwhile, heat a large skillet or griddle over high heat. Preheat the grill or a grill pan.
Toss the onions and peppers with 1 teaspoon of the blackening seasoning and some salt and black pepper. Add the oil to the large skillet and cook the onions and peppers, about 2 minutes. While the vegetables are cooking, dust both sides of each fillet with salt, black pepper and the remaining blackening seasoning, just enough to coat. Flip the vegetables and continue cooking until lightly browned, about 3 minutes longer, and then push to one side of the skillet. Cook the fish on the other side of the skillet, 2 minutes per side, flipping once halfway through. Divide the fish and vegetables between the 2 small hot skillets. There should be a lot of steam and sizzling!
Dip the limes in blackening seasoning, grill 1 to 2 minutes, and then squeeze lime juice over each fillet.
Serve the fajitas in the cast-iron skillets with an oven mitt (very hot!). Assemble in the warmed tortillas, and top with pico de gallo, shredded cheese, lettuce and tomatoes, if desired.
This recipe was provided by a chef, restaurant or culinary professional and may have been scaled down from a bulk recipe. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.