1. Make the crust:

1. Preheat your oven to 325°F (163°C).
2. In a mixing bowl, combine the graham cracker crumbs, melted butter, and sugar (if using) until the crumbs are evenly coated.
3. Press the mixture firmly into the bottom of a 9-inch (23cm) springform pan to form an even layer.
4. Bake the crust for 10 minutes, then set aside to cool.

2. Prepare the cheesecake filling:

1. In a large mixing bowl, beat the softened cream cheese and sugar together until smooth and creamy.
2. Add the eggs one at a time, mixing well after each addition.
3. Mix in the sour cream, heavy cream, vanilla extract, lemon juice, and lemon zest until fully combined.
4. Pour the filling over the cooled crust and smooth the top with a spatula.

3. Bake the cheesecake:

1. Place the springform pan in a larger baking dish and add hot water to the larger dish, creating a water bath (this helps prevent cracks).
2. Bake the cheesecake for 60-70 minutes, or until the edges are set but the center is slightly jiggly.
3. Turn off the oven, crack the door open slightly, and let the cheesecake cool inside the oven for 1 hour.
4. Remove from the oven, run a knife around the edges to loosen, and cool completely at room temperature. Then refrigerate for at least 4 hours, or overnight.

4. Make the lemon curd:

1. In a small saucepan, whisk together the lemon juice, lemon zest, sugar, and egg yolks.
2. Cook over medium heat, stirring constantly, until the mixture thickens and coats the back of a spoon (about 5-7 minutes).
3. Remove from heat and stir in the butter until smooth.
4. Let the lemon curd cool completely before spreading it over the chilled cheesecake.

5. Assemble the cheesecake:

1. Spread the cooled lemon curd evenly over the top of the cheesecake.
2. Refrigerate for an additional 30 minutes to allow the topping to set.