1. To make batter: In a large bowl, combine flour, cornstarch, baking powder, and salt. Blend egg and beer, then quickly stir into the flour mixture (a few lumps are ok).
2. 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, oregano, cumin, dill and cayenne.
3. Heat oil in deep fryer to 375 degrees F.
4. Dust fish lightly with flour. Dip into beer batter and fry until crisp and golden brown; drain on paper towels.
5. Lightly fry tortillas.
6. To serve; place fried fish in a tortilla and top with cabbage and white sauce.