Mix lemon juice with soy milk and apple cider vinegar. Let it sit until it curdles. In a big bowl, cream the butter and whisk in the sugar. Once the ingredients have combined completely, add the lemon zest.
In another bowl mix flour, poppy seeds, baking powder and salt.
Stir the dry ingredients into the butter-sugar mixtures spoon by spoon, alternating with the lemon-milk mix.
Finally, add the sparkling water to the batter. Try not to overmix it so that the dough will stay light and fluffy.
Pour the batter into the loaf pan and place it into the oven. Bake the cake for about 55 to 60 minute or until a wooden skewer comes out with only a few crumbs attached to it. Once the cake is ready, take it out of the oven, let it cool in the pan for a few minutes before placing it onto a cooling rack.