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.