About 1 hour before you plan to fry the fish, make the batter. Mix flour and 1 teaspoon salt in a mixing bowl. In another bowl, stir together egg yolk, beer, and milk. Pour this into the dry ingredients, stirring (but not beating) the mixture well with a wire whisk. Cover and chill 30 minutes.
Cut the potatoes into 3- by 3/8-inch sticks and place them in a bowl. Cover with cold water, and stir them a bit to help remove the excess starch.
Pour oil to a depth of 3 or 4 inches in a stockpot or Dutch oven; place it over medium-high heat, and heat it to 375°. Preheat the oven to 200°. Place a wire rack on a baking sheet and set it in the oven.
Drain the potatoes well, rinse them under running water, and lay them out on a terry cloth towel, patting them completely dry. When the oil reaches 375°, fry the potatoes, in batches, until they are thoroughly browned, 8 to 10 minutes. Do not crowd the pot, and carefully maintain the temperature at 375°. As they are finished, transfer the potatoes to the wire rack to drain and stay warm.
While the last batch of potatoes is frying, beat the egg white to soft peaks and gently but thoroughly fold it into the batter. Rinse the fish fillets and pat them dry. Using tongs, drop several pieces of fish into the batter, then transfer them to the hot oil to fry. Each batch will take about 4 minutes. Remember to maintain the heat at 375°. Serve hot, and pass malt vinegar and salt.