Sift the flour into a large bowl and add the egg yolk. Gradually whisk in the lager, then add the measured water and whisk until well combined.
Whisk the egg white in a separate bowl until stiff peaks form. Fold this into the batter.
Fill a deep-fat fryer or a large, deep, heavy-based saucepan two-thirds full with vegetable oil. Heat the oil to 180°C (350°F) or until a cube of bread turns golden in 10–15 seconds.
Cut the haloumi into 1 cm (1/2 in) slices, then dip in the batter to coat. Fry the haloumi in batches for 3–4 minutes, or until crisp and golden-brown. Remove with a slotted spoon, season and serve on rocket leaves with wedges of lemon to squeeze over.