In a mixing bowl, combine the flour, cornstarch, baking powder, salt, and black pepper. Gradually whisk in the cold sparkling water until a batter forms.

Heat Oil

Heat vegetable oil in a deep frying pan over medium-high heat.

Fry the Fish

Dip each fish fillet into the batter, allowing excess to drip off, then carefully place it in the hot oil. Fry for about 4-5 minutes on each side, or until golden brown and crispy.

Drain Excess Oil

Remove the fish from the oil and place on a plate lined with paper towels to drain excess oil.

Tamarind Tartar Sauce Preparation

Make the Sauce

In a small bowl, mix together the mayonnaise, tamarind paste, Dijon mustard, chopped capers, lemon juice, salt, and black pepper until well combined.

Sandwich Assembly

Prepare Avocado

Slice the avocado and mash it lightly with a fork, seasoning with salt and pepper to taste.

Spread Sauce

To assemble the sandwich, spread a generous amount of tamarind tartar sauce on the bottom half of each challah bun.

Assemble the Sandwich

Add a layer of shredded iceberg lettuce, followed by the fried fish fillet, mashed avocado, and the top half of the challah bun.