Preheat your oven to 350°F (175°C) and grease a 9-inch springform pan thoroughly. Combine graham cracker crumbs, sugar, and melted butter in a bowl, then press the mixture firmly into the bottom of the prepared springform pan.

Bake the crust for 10 minutes, then remove it from the oven and set it aside to cool slightly while you prepare the filling.

Reduce oven temperature to 325°F (160°C). In a large bowl, beat the softened cream cheese with an electric mixer until smooth, then gradually add sugar and flour, mixing until just combined and scraping down the sides of the bowl.

In a separate bowl, whisk together the eggs and egg yolk, then slowly add them to the cream cheese mixture, mixing on low speed until fully incorporated.

Stir in the heavy cream, sour cream, vanilla extract, and lemon zest, mixing until the batter is smooth and lump-free.

Pour the cheesecake batter over the cooled crust in the springform pan, smoothing the top with a spatula.

Prepare a water bath by wrapping the outside of the springform pan tightly with several layers of aluminum foil and placing it in a larger roasting pan; fill the roasting pan with hot water halfway up the sides of the springform pan.

Bake for 1 hour and 15 minutes to 1 hour and 30 minutes, or until the edges are set but the center still slightly jiggles when gently shaken; turn off the oven and let the cheesecake cool in the oven with the door ajar for 1 hour.

Remove the cheesecake from the water bath and aluminum foil, then let it cool completely on a wire rack at room temperature; once cooled, refrigerate for at least 6 hours, or preferably overnight, before serving.