Marinate the halloumi in the buttermilk for at least 20 minutes.
To make the batter, sift the flour into a large bowl and add the egg yolk. Whisk in the lager gradually and add 125ml/4½oz ice-cold water until well combined.
Whisk the egg white until stiff peaks form in a separate bowl. Fold the egg whitesinto the batter and chill in the fridge until required.
Fill a deep-fat fryer or a large, deep, heavy-bottomed pan two-thirds full with vegetable oil. Heat the oil to 180C/350F (use a probe thermometer to check the temperature if you're not using a deep-fat fryer). (CAUTION: Hot oil can be dangerous. Don't leave unattended)
Shake any excess buttermilk off the halloumi, then dip into the flour and coat in the batter. Fry the halloumi 5 minutes, or until crisp and golden-brown. Remove with a slotted spoon and drain on a baking sheet lined with kitchen paper. Season to taste with salt and freshly ground black pepper.
For the röstis, boil the potatoes in for 7-8 minutes, then drain. Once cool enough to handle, peel and grate coarsely, and season with salt and freshly ground black pepper.
Form flat, rounded shapes of the potato mixture and chill in the fridge for 20 mnutes.
Heat the oil in a frying pan and fry the rostis on a medium heat for 4-5 minutes on each side, or until crisp and golden-brown. Drain on kitchen paper.
For the tartare sauce, stir the capers, gherkins, dill and lemon in a bowl, add the mayonnaise and mix until well combined.
Serve the halloumi on top of a rosti with a spoonful of tartare sauce and a lemon wedge on the side.