Preheat oven to 325 degrees F (165 degrees C), moving a rack to the middle of the oven. Mix ricotta and cream cheese with an electric mixer in a large bowl on medium speed until very smooth and uniform, for about one or two minutes. Make sure there are little to no lumps before adding the remaining ingredients.
Add sugar, eggs, vanilla, butter, lemon zest and juice and beat until just fully incorporated. Add the cornstarch and flour afterward until just mixed in; you don’t want to beat the mixture too much.
Grease a 9 inch spring-form pan with shortening or baking spray and triple wrap the outside and bottom with foil. Pour cheesecake batter into the spring-form pan and tap it multiple times onto the counter top from a low height, at least 10 times to remove any bubbles in the cheesecake.
Use a 9×13 Pyrex dish or large roasting pan and fill it with 1 inch of water to create a water bath for your cheesecake to sit in. Place the spring-form pan into the water bath dish and carefully put into your preheated oven on the middle rack.
Bake for 1 hour and 15 minutes; afterwards, turn the heat off and crack open the oven door for about another hour. The ricotta cheesecake needs this time to cool slowly, removing it from the oven quickly will deflate it and cause it to crack. Remember: "you want a jiggle, but just a little, in the middle," when it's done.
Refrigerate for at least four hours, preferably overnight, before removing from spring-form pan and serving chilled.
Keep in fridge in airtight container or wrapped with plastic wrap for up to 1 week.