Preheat oven to 275°F. Set a rack in the center of the oven with a baking sheet lined with parchment.
Heat 3 inches of oil in a tabletop fryer or deep Dutch oven over medium to medium-high heat to 365°F.
Whisk up Old Bay, flour, garlic powder, salt and pepper in one bowl then whisk together the beer and egg in another bowl. Add wet ingredients to dry and combine.
Pat cod dry and wait for oil to come to temperature.
Whisk up malt vinegar, sugar, olive oil, salt, pepper and celery seed. Add cabbage, carrot and onion, and toss to coat.
Dip fish to coat in batter then add to hot oil. Fry in 2 batches, keeping finished pieces warm in the oven.
Plate fish on rolls lined with bread-and-butter pickles and topped with malt slaw. Serve chips or oven fries alongside.