Preheat the oven to 350 degrees F.
Combine the flour, the chili powder and 1 teaspoon of the pepper in a shallow dish. Combine the beer and mustard in a separate shallow dish. Mix the panko, granulated garlic, paprika, salt and remaining pepper into a third shallow dish.
Dredge the fish in the flour mixture and then the beer and mustard and lastly in the panko mixture. Arrange the fish on a sheet pan, lined with a cooling rack, and refrigerate until ready to cook.
For the slaw: In a large bowl, whisk together the mayonnaise, sour cream, lime zest and juice, chipotle, sugar, vinegar and salt and pepper, to taste. Fold in the cabbage, onion, bell peppers, scallions, and cilantro and adjust the seasoning, if necessary. Refrigerate.
Spread the butter on inside of rolls and arrange, crust side down, on a rack in the oven to toast while fish cooks.
Heat 1 cup oil in a cast iron or heavy bottom skillet over medium-high heat. Carefully add the fish, cooking on both sides until the coating turns golden brown, about 2 to 3 minutes. Remove to a cooling rack to drain. Keep warm in the oven, if necessary.
To make each sandwich, arrange a piece of fish on the bottom half of the rolls and top with the slaw. Cover with another piece of bread and serve.
For an alternate method: Prepare fish as above, but arrange on a sheet pan lined with a baking rack. Put the fish into the hot oven and bake until cooked through, 15 to 18 minutes. Briefly broil both sides of fillets to get a good crisp coating.
Recipe courtesy of Guy Fieri