Pre-heat oven to 400F and place 1 cup of grapes on lightly oiled baking sheet. Once the oven is up to temp, pop the grapes in for about 10 mins until they start to burst slightly. Remove from oven to cool.
Reduce oven temp to 350F and brush 9in springform pan with olive oil.
Using an electric mixer, beat the eggs yolks and sugar until pale and thick. Add in the olive oil, citrus zest and juice. Stir in flour and mix until just combined.
Beat or whisk eggs whites and salt until light stiff peaks form. Gently fold egg whites into flour mixture.
Pour batter into pan and drop roasted grapes in, gently submerging them slightly with your fingers.
Bake for 10-15 min, remove from oven and add remaining grapes, nestling them just a bit into the top of the batter.
Return to the oven for another 30-40 or until a tester skewer comes out clean. Meanwhile, put sugar crystals in a ziploc bag, cover with a kitchen towel and break up by pounding lightly with a rolling pin or mallet.
Brush warm cake with olive oil and top with crushed sugar crystals or sugar of choice. Allow to cool completely before removing spring and slicing.