For the crust:
For the filling:
For the Lemon Curd
1 1/2 cups sugar
1/4 pound unsalted butter, room temperature
4 extra-large eggs
1/2 cup lemon juice (3 to 4 lemons)
1/8 teaspoon kosher salt
Preheat the oven to 325°F.
Prepare a 9-inch springform pan for a water bath. Place a large square of heavy-duty aluminum foil underneath the pan. Gently fold the edges up and around the pan. Repeat twice so there’s three sheets of foil to ensure a waterproof seal. Gently fold the top of the foil around the edge of the pan. Spray with nonstick cooking spray. Make sure there is no way that the water will get over the foil or your crust will get soggy.
Make the crust:
In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter until the mixture is moistened. Press firmly into the bottom and quarter way up the sides of the prepared pan. Bake for 10 minutes. Place on a wire rack to cool. Maintain oven temperature.
Make the filling:
In a large bowl, use an electric mixer to beat the cream cheese on medium speed until completely smooth and free of lumps, scraping down the bowl as needed. Add the sugar and beat until combined. Add in the sour cream, lemon juice, lemon zest, and flour. Add the eggs and yolk, one at a time, and beat until just combined. Be careful not to overmix.
Pour the batter into the foiled-wrapped springform pan. Tap the pan against the counter a few times to release any air bubbles in the batter.
Prepare the water bath:
Place the pan in a larger roasting pan. Pour boiling water into the roasting pan until the water is about halfway up the sides of the cheesecake pan.
Bake the cheesecake:
Bake at 325°F for 1 hour 15 minutes, or until the top looks slightly dry. If the middle is slightly wobbly, that’s fine. Err on the side of underbaking rather than overbaking.
Turn off the oven heat and open the oven door open just by 1-inch. Let the cheesecake cool inside the oven for 1 hour. Don't skip this part. This will prevent cracks from forming.
Make the Lemon Curd
Using a carrot peeler, remove the zest of 3 lemons, being careful to avoid the white pith. Put the zest in a food processor fitted with the steel blade. Add the sugar and pulse until the zest is very finely minced into the sugar.
Cream the butter and beat in the sugar and lemon mixture. Add the eggs, 1 at a time, and then add the lemon juice and salt. Mix until combined.
Pour the mixture into a 2 quart saucepan and cook over low heat until thickened (about 10 minutes), stirring constantly. The lemon curd will thicken at about 170 degrees F, or just below simmer. Remove from the heat and cool or refrigerate.
Remove the cheesecake from the oven and from the water bath, unwrap the foil, and transfer it to a cooling rack. Run a thin-bladed flexible knife around the edge of the cake to make sure it's not sticking to the sides (which can cause cracks as it cools). Let the cheesecake cool completely to room temperature on the rack.
Refrigerate until completely chilled, at least 5 hours or up to 3 days. Store the cheesecake uncovered to avoid creating condensation, which will create a mushy texture.
Release the cheesecake from the pan onto a cake stand or serving plate and top with the lemon curd. Slice using a big sharp knife, wiping it off under running hot water in between cuts. Serve.