To make beer batter: In a large bowl, combine flour, cornstarch, baking powder, and salt. Blend egg and beer, then quickly stir into the flour mixture (don't worry about a few lumps).
To make white sauce: In a medium bowl, mix together yogurt and mayonnaise. Gradually stir in fresh lime juice until consistency is slightly runny. Season with jalapeno, capers, oregano, cumin, dill, and cayenne.
Heat oil in deep-fryer to 375 degrees F (190 degrees C).
Dust fish pieces lightly with flour. Dip into beer batter, and fry until crisp and golden brown. Drain on paper towels. Lightly fry tortillas; not too crisp. To serve, place fried fish in a tortilla, and top with shredded cabbage, and white sauce.