Fried-Shrimp Po'boys

Preheat oven to 375 degrees F. In a large cast-iron skillet, or heavy pot, fitted with a candy thermometer, heat oil to 325 degrees F. Meanwhile, in a shallow bowl, whisk together egg, milk, 1 cup water, and mustards; set aside. In a medium bowl, combine flour and garlic salt. Season with salt and pepper and set aside. Slice bread loaves lengthwise and place, crust down, on a large baking sheet. Transfer to oven, turn off heat, and warm bread until crispy. Meanwhile, working in batches, dip shrimp in egg batter, then dredge in flour mixture. Fry shrimp in heated oil, until they're cooked through and float to surface, 2 to 3 minutes per batch. Transfer to a paper-towel-lined plate to drain. Spread bottom half of each warmed loaf with 1 tablespoon tartar sauce. Divide lettuce, tomatoes, and shrimp among sandwiches. Drizzle 1 tablespoon ketchup and a few dashes of hot sauce, if desired, over shrimp. Top with remaining bread halves.