Stir together mayonnaise, lemon juice, and lemon zest in a small bowl until smooth.
Combine cabbage, vinegar, ½ teaspoon of salt and ¼ teaspoon of pepper in a large bowl, tossing to coat. Set aside.
Whisk together fl our, garlic powder, paprika, remaining salt and remaining pepper in a shallow dish. Place eggs and breadcrumbs in 2 separate shallow dishes.
Heat oil in a deep skillet or dutch oven over medium heat to about 375°F.
Meanwhile, dredge each halibut fi llet in fl our mixture, dip into beaten eggs, and coat with panko breadcrumbs. Place onto a baking sheet lined with parchment paper.
Fry coated fi llets until golden-brown and crispy, about 3-4 minutes, fl ipping halfway through. Remove from oil and place onto a paper-towel-lined plate to drain.
Coat pumpernickel buns with lemon aioli. Top with one halibut fi llet, pickles and a generous portion of cabbage slaw. Serve with fries.
