¾ cup mayonnaise
3 tablespoons chopped fresh tarragon
1 tablespoon malt vinegar or apple cider vinegar
1 cup all-purpose flour, plus more for dredging
1 ½ teaspoons Old Bay seasoning
1 (12 ounce) bottle lager beer
Peanut oil, for frying
3 (8-10 ounces) russet potatoes, scrubbed
1 ½ pounds halibut fillets
1 or 2 large dill pickles cut into ¼ inch-thick rounds
Kosher salt and freshly ground black pepper
In a small bowl, whisk together the mayonnaise, tarragon and vinegar and then season with salt and pepper. Cover with plastic wrap and put the mixture in the refrigerator until ready to serve.
Whisk the flour and Old Bay together in a large bowl. Pour in the beer and whisk until the batter is smooth.
Fill a large, heavy saucepan half-full with oil and heat to 350F. Heat the oven to 200F.
While the oil heats, slice the potatoes into 1/4-inch thick rounds. Working in batches, add the potato slices to the oil. Cook, turning occasionally, until the potatoes are tender, cooked through and just starting to turn golden, about 5 to 8 minutes. Transfer to a plate lined with paper towels to drain and set aside.
Cut the fish crosswise into ½ to ¾ inch thick slices and season with salt and pepper on all sides. Place some flour in a shallow bowl. Dredge the fish in the flour. Working in batches and bringing the oil back up to 350F between each batch, dip the fish into the beer batter to coat, let excess batter drip back into the bowl and use a bamboo skewer or your hands to lower the fish halfway into the oil. Hold for 10 seconds, then release (this helps prevent sticking on the bottom). Fry until deep golden and just cooked through, 4 to 5 minutes. Transfer the fish to a rack set over a baking sheet and slide into the oven to keep warm.
When you’ve finished frying the fish, heat the oil to 375F. Again working in batches, fry the potatoes a second time until puffed and golden brown, 2 to 3 minutes. Pile them in a bowl lined with paper towels.
Working in batches, dredge the pickles in flour, dip into the beer batter to coat lightly, let the excess batter drip off and fry until golden, 3 to 4 minutes. Drain on paper towels.
Divide the fish, chips and pickles among your plates and serve immediately with the tarragon- malt vinegar mayonnaise on the side.