1. Preheat oven to 350° F. Slice the citrus very thin. Remove any seeds. Line a 9-inch cake pan with Simply Done® Parchment Paper. Spray the pan with nonstick spray. Melt 2 tablespoons of butter, and pour into the pan. In a single layer, sprinkle 1/4 cup of sugar over the melted butter. Begin arranging citrus slices in a random pattern in the bottom of the pan. Lay the smaller slices first. Overlap with larger slices until all parchment paper is covered. Cut some slices in half, if necessary.
2. Using an electric mixer in a large mixing bowl, beat the remaining butter and sugar until pale and fluffy. Add the egg yolks one at a time, fully incorporating each yolk before adding another one. Slowly mix in the almond flour, cornstarch, baking powder and salt. Add the orange juice, orange zest and lemon zest.
3. In another bowl, whip the egg whites to stiff peaks. Add 1/3 of the egg whites to the batter. Stir to incorporate. Fold in the remaining 2/3 of egg whites. Be careful not to lose volume in the egg whites. Spoon the batter into the cake pan, and level the top. Bake for 50 minutes or until the cake is cooked in the middle.
4. Let cool in the pan for 10 minutes. Run an offset spatula around the edges of the pan, and invert onto a cake plate. Gently remove the parchment paper to reveal the top.

Per Serving: Calories: 430, Fat: 28 g (10 g Saturated Fat), Cholesterol: 155 mg, Sodium: 190 mg, Carbohydrates: 36 g, Fiber: 5 g, Protein: 12 g.