Half-fill a medium saucepan with vegetable oil and place over high heat (see note). Heat the oil to 180°C (350°F) – check with an oil thermometer. (For a guide to testing the oil temperature without a thermometer, see note.)
In a large, shallow bowl, whisk the egg and milk together to make an egg wash. Place the rice flour in another bowl and the panko breadcrumbs in a third. Dust each ling fillet in the rice flour first, then dip it in the egg wash and, lastly, the breadcrumbs.
Tap off any excess crumbs and lower the ling fillets carefully into the hot oil. Fry the ling for about 2 minutes, or until crisp and golden. Remove from the saucepan and drain on a baking tray lined with paper towel.
Place the bottom half of each brioche bun on a tray. Add two lettuce leaves to each and top with the fried fish. Add the pickled onion rings, pickled cucumber and tartare sauce, then place the other brioche half on top and serve.
• For this much fish, it’s safe to use a 6 litre (203 fl oz/24 cup) saucepan. When deep-frying in a saucepan, it’s advisable to keep the oil level a good 10 cm (4 in) below the rim of the pan. The other option for deep-frying is, of course, an electric deep-fryer. The used oil can be strained, cooled and refrigerated for later use.
• To test the temperature of the oil, drop a cube of bread into it. If it turns golden in 15 seconds, it’s sitting perfectly on 180°C (350°F). If the bread takes only 10 seconds to turn golden, then the oil is around 190°C (375°F). If it takes 20-30 seconds to brown, the oil is too cold. Be patient and get the oil to the correct temperature before proceeding.