Preheat oven to 300 degrees F. Grease a 9-inch round cake pan and line with parchment paper. Grease the parchment paper and coat with flour. This will prevent the orange slices from sticking to the bottom of the pan.
Stir water and sugar together to form a slurry. Pour into the bottom of the cake pan. Arrange orange slices in a single layer in the bottom of the pan. Set aside.
In a stand mixer with beater attachment, cream together butter and sugar. Add egg yolks, one at a time, and beat until combined. Beat in ricotta and orange juice and zest for about 5 minutes, or until ricotta becomes smooth.
Mix in almond flour, cornmeal, and salt until just combined. Batter will be very clumpy.
Clean stand mixer and using whisk attachment, whip egg whites until stiff peaks form. Gently fold egg whites into batter.
Scoop batter over the orange slices. Gently drop cake pan on countertop a few times to even out the batter, but be careful not to disturb the orange slices. Bake for 55-60 minutes, or until toothpick inserted into the center of the cake comes out clean.
Remove cake from the oven. Allow the cake to cool in the cake pan for 20 minutes. Run a knife around the edges of the pan to loosen. Carefully invert the cake unto a serving plate. If any slices stick to the pan, just gently rearrange them. Serve at room temperature or refrigerated. Enjoy!