1) Preheat oven to 180°C/ gas mark 4. Butter and flour a 20-cm round cake pan.
2) In a large bowl, using an electric mixer, beat the egg whites to soft peaks. Add half of the sugar and beat until the mixture holds stiff peaks.
3) In a separate large bowl, beat the vegetable oil, the remaining sugar and salt. Add the egg yolks one at a time. Add the chopped mint, lemon juice and lemon zest. Add the flour and mix until just combined. Set aside.
4) Slowly add half of the egg white mixture into the oil mixture. Using a rubber spatula, carefully fold in the remaining egg white mixture. Spread the batter into the prepared cake pan and bake for 40 to 45 minutes, until a toothpick inserted into the middle of the cake comes out clean. Transfer the cake to a cutting board and allow to cool before slicing.
5) To make the syrup, in a small saucepan, combine the sugar, water, lemon juice and lemon zest over medium heat. Bring to a boil, then reduce the heat and simmer until the sugar has dissolved, about 5 minutes, stirring occasionally. Remove the pan from the heat and allow the syrup to cool, about 20 minutes.
To serve, spoon some of the syrup onto each small plate or shallow bowl and lay a slice of cake on top. Drizzle the cake with additional syrup and garnish with a small sprig of mint.