Combine first 4 ingredients in a shallow dish. Dredge fish in flour mixture, shaking off excess.
Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add fish to pan; cook 3 minutes on each side or until fish flakes easily when tested with a fork.
While fish cooks, combine mayonnaise, relish, and red pepper. Spread about 1 teaspoon mayonnaise mixture over cut side of each roll half. Layer 2 tomato slices, 1 onion slice, and 1 fillet on bottom of each roll; top with top halves of rolls.
We loved these. I did add some cajun seasoning to the dry mixture that you dredge the fish in. I think it was Tony Cachere's, but it might have been Emeril's. Probably about 1/2-1 tsp. I think that made a big difference in the flavor of the fish, although the fish itself was amazingly fresh so I think it would have been difficult to mess up any way I cooked it.
So many flaws with this recipe. First off, a kaiser roll is just not a convenient thing to eat this on - a more unorthodox, but more practical alternative, would be a hoagie roll or good hot dog bun. Spread the mayo on, layer with tomato and onion and top with the fish. Sort've like a po'boy. It accommodates the shape of a 6-oz. fillet much better. Otherwise, this thing lacks structural integrity and becomes a total messy train wreck. I found the cooking time to be completely wrong. A 6-oz. fillet won't cook through in that amount of time. I ended up doing 3 minutes on all four sides. Of course it could be that my fillets were just thicker, but honestly, they all looked about like mine at the Whole Foods counter. I'd say a cheaper and easier alternative would be to do this recipes with catfish fillets, which are also more readily available, at least in the DC suburb where I live. Otherwise it's simple and tasty, although pretty darn unremarkable.
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