Cod in Beer Batter with Tartar Sauce

Name: Cod in Beer Batter with Tartar Sauce
Ingredients: cooking oil (for deep-frying ( about 1 quart)); 1 cup all-purpose flour; 1/4 cup cornstarch; 1/2 teaspoon salt; 1 egg (large, lightly beaten); 1 cup beer (or as needed); 2 pounds cod (haddock or hake fillets, cut into approximately 1 1/2 x 3 pieces); 1 cup mayonnaise; 1 tablespoon Dijon mustard; 1 scallion (both white and green parts, minced); 1 1/2 teaspoons lemon juice; 2 tablespoons dilled gherkins (minced); 1 tablespoon capers (minced, drained); 2 tablespoons parsley (snipped fresh); 1 pinch salt; 1 pinch black pepper