PreparationMake the slaw: Combine lemon and lime juice, hot sauce, olive oil, oregano, and salt in a large bowl. Add onion, jalapeño, and poblano chile and toss to combine. Let marinate at least 30 minutes and up to 1 hour while you prepare the rest of the dish. Make the tartar sauce: Combine cornichons, mayonnaise, parsley, capers, tomato powder (if using), vinegar, chili powder, and salt in a small bowl. Make the fish: Whisk flour, cornstarch, oregano, onion powder, tomato powder (if using), garlic powder, and 3 tsp. salt in a large bowl. Add club soda and quickly whisk to incorporate. Pour canola oil into a large skillet to a depth of 1/4". Heat oil over medium-high until shimmering, hot, and an instant-read thermometer registers 350°F. Dip fish into batter to coat, shake off excess, then fry until golden, crispy, and cooked through, about 4 minutes per side. Work in batches if necessary and adjust the heat as needed to keep oil at 350°F. Transfer to paper towels and season with salt. Assemble the sandwiches: Strain the slaw and discard liquid. Transfer slaw to a medium bowl. Spread 1 Tbsp. tartar sauce on both sides of each bun and place tomato slices on bottom halves. Divide fried fish and slaw among sandwiches and add top halves of buns. Do Ahead Slaw can be made, strained, and refrigerated up to 1 day ahead. Tartar sauce can be made and refrigerated up to 2 days ahead. Cooks' NoteTomato powder can be purchased at specialty food stores and online.