Do ahead: In a cheesecloth lined strainer placed over a bowl, strain the ricotta for 12 hours or overnight in the refrigerator.
To make Candied Lemons (if using): Thinly slice the lemons and remove any pits. Fill a large bowl with ice and cold water. Set aside. Bring a medium pot of water to a boil and boil lemon slices for one minute, then remove to the ice water until completely cooled, then drain the water/ice. In a medium-sized skillet, bring white sugar and water to a simmer, stirring until sugar is dissolved. Add lemon slices in a single layer and gently simmer, turning a few times, for about 1 hour, or until rinds are translucent. Remove lemon slices to a cooling rack to cool completely.
When ready to bake, preheat oven to 325 Butter an 8-inch springform pan and line the bottom with parchment paper.
In a large bowl with an electric mixer or the bowl of a stand mixer with the paddle attachment, combine the ricotta, 3/4 cup white sugar, flour, egg yolks, lemon zest and juice until smooth. Set aside or if using stand mixer, remove to a large bowl and clean bowl.
In another bowl (or clean stand mixer bowl, fitted with a whisk), beat the egg whites with 1/4 sugar until stiff peaks form.
Use a spatula to gently fold the egg whites in to the ricotta mixture. Pour in to prepared pan.
Place in to preheated oven and bake for 50-55 minutes, or until it is golden and set. Remove to a cooling rack to cool in the pan. Cake will sag or sink in the middle. Once cooled, leave in pan, cover with plastic wrap and refrigerate 4 hours or overnight.
When ready to serve, run a knife around the outside of the pan and unmould. Dust with icing sugar and top with Candied Lemons or the topping of your choice.