Prepare the candied lemon slices. Slice the lemon into ⅛ inch- thick rounds, discarding any seeds. Combine the sugar, lemon juice and water in a large non-stick skillet over medium heat. Bring to a boil and reduce heat to low. Add the lemon slices in one layer and simmer, turning the lemon slices occasionally. Cook until the lemon rinds soften, 15 minutes. Remove from heat and transfer the slices onto a parchment paper sprayed with cooking spray. Let cool.
Preheat oven to 350 degrees F. Spray 9-inch cake pan with cooking spray and line the bottom with parchment paper.
Combine flour, sugar, salt, baking soda and baking powder in a large bowl. In a separate bowl, whisk together olive oil, buttermilk, eggs, lemon zest, and lemon juice. Add the wet ingredients to the dry, stirring just until combined. Pour the batter into the prepared cake pan and bake for 45 minutes - 1 hour or until a toothpick inserted comes out clean. Remove from oven and let cool on the wire rack.
Garnish cake with lemon slices and serve.