Heat the oil in a large, deep pot over medium heat until it reaches 375 degrees F on a candy thermometer. Preheat the oven to 300 degrees F.
In a medium bowl, combine the flour, seafood seasoning, baking soda and a pinch of salt. Make a well in the center of the flour mixture and gently stir in the beer. Mix with a fork until just combined. Dip the fish fillets in the batter, coating the fish generously.
Fry the fish: Start by dipping about one-third to one-half of a fillet into the oil and wait for the batter to start puffing before sliding the rest of the fillet in. Fry the battered fillets until golden brown and crispy, turning once, about 5 minutes. Drain on paper towels, blotting excess oil. Arrange the fish on a baking sheet lined with a cooling rack, and hold in the oven until serving.
Slice the po'boy rolls lengthwise almost three-quarters of the way through and scoop out some of the insides. Schmear the rolls with tartar sauce. Add the fried fish and top with some lettuce, pickles, tomatoes and red onion. Slice the sandwich in half on the bias before serving.
Combine the mayonnaise, shallots, cornichons, mustard, celery salt, cayenne and malt vinegar in a bowl. Season with salt, if needed. Cover and refrigerate until ready to use.