Peel the apples, remove the core with a corer and cut the apples into thick slices. Drizzle with lemon juice.
For the batter: Separate the eggs. Split the vanilla bean lengthwise and scrape the seeds into a bowl along with the flour, milk, mineral water, oil, egg yolks and lemon zest. Beat the egg whites with salt and 2 tablespoons sugar until stiff and fold into the batter.
In a deep saucepan or fryer, heat the oil to 160°C (approximately 325°F). If a crumb of bread is added to the oil and it sizzles, the oil is ready.
Dip the apple slices into the batter and working in batches, fry the apples until golden brown.
Remove with a skimmer and drain on paper towels. Sprinkle with sugar and serve hot.