Preheat the oven to 350°F (180°C) and grease a 9x5 inch (25x13cm) loaf pan with oil. Then line the pan with a strip of parchment paper. Set the pan aside.
To a medium-size bowl, add all-purpose flour, cornstarch, baking powder, baking soda, and a pinch of sea salt. Mix with a whisk and set aside.
In a large mixing bowl, using a whisk, mix white granulated sugar, sunflower oil, and vegan sour cream until combined. Then add lemon juice, unsweetened almond milk, and 5 drops of lemon essential oil and mix again until combined.
Sift half of the dry ingredients into the wet ingredients, mix, sift the second half of the dry ingredients, add lemon zest and poppy seeds, and stir together until just combined. Be careful not to overmix the batter (or the cake will get too dense).
Pour the batter into the prepared baking pan and spread it evenly. Bake it in preheated oven for 50-55 minutes. The cake is done when a toothpick inserted in the middle comes out clean. If it looks wet, bake for additional 5 minutes.
Remove the cake from the oven and let cool for about 15 minutes. Then carefully remove the cake with the parchment paper flaps on the sides and place it on a cooling rack to let cool completely.
Make the lemon glaze by mixing powdered sugar and lemon juice until it forms into a runny glaze. Pour the glaze over the cooled cake and spread evenly.
Slice and serve!