For the crumb crust:

1. Preheat the oven to 325°F/170°C.
2. Have ready a 9-inch round cake pan with removable bottom (springform pan). Line it with foil if you want to (see Notes below).
3. Grind graham crackers in the food processor until you have very fine crumbs.
4. Add the melted butter and pulse until combined and no dry spots remain. It will look like wet sand.
5. If you already have the crumbs, mix the ingredients in a bowl. 
6. Scatter this mixture in the prepared pan and press firmly onto the bottom. Use your hands or a measuring cup. 
7. Bake for 10 minutes. Remove from the oven and place on a cooling rack while making the filling.
8. Don't turn off the oven. 

For the ricotta filling:

1. Lower the oven to 275°F/140°C.
2. Mix cream cheese, ricotta, and sour cream in a large mixing bowl until very smooth. You can use a handheld mixer at low speed, or a wire whisk. If the cheese is at room temperature, mixing it by hand should not be a problem. We don't want to incorporate air into the batter. 
3. Gradually add sugar and mix until it's well incorporated. If using a handheld electric mixer, use it only until this part and at low speed. I use a whisk.
4. Add eggs, two at a time, mixing with a wire whisk until fully combined, scraping down the sides of the bowl a few times. Be patient as the egg white sometimes takes a little longer to incorporate, and don't be tempted to beat. This is good for creaminess later.
5. Add vanilla and lemon zest, stirring to incorporate well. The batter can have some ricotta pieces and that is fine. Especially if you only use a hand whisk. 
6. Pour the mixture into the baked crust. It might be warm and that’s fine.
7. Bake the cheesecake for about 1 hour and 20 minutes, until the top is dry and the filling barely jiggles in the center. It might take you more or less time so start checking 10 minutes before the given time. Don't be tempted to bake it until the center is completely set. It will solidify as it cools down and after it’s refrigerated. 
8. Cool completely to room temperature in the pan on a wire rack, the center will fall a little. 
9. Wrap in plastic (still in the pan) and refrigerate for at least 6 hours. I usually leave it for 1 day. It's essential to chill it for several hours before eating it.
10. When ready to serve, run a smooth-bladed knife carefully around the edges to loosen the cheesecake, open the springform ring, and, using a spatula, lift and transfer the cake to the serving plate.
11. Serve cold or at room temperature, plain or with a topping. We love fresh berries that are tangy and make a great contrast with the creamy filling, especially raspberries. 
12. Store leftovers covered in plastic wrap or in an airtight container in the refrigerator for 4-5 days or frozen for up to a month. For the freezer, I wrap it in cling film first and then aluminum foil. Defrost at room temperature or in the fridge, depending on how much time you have.