To make the crust, preheat the oven to 325°F (165°C). In a food processor, pulse the gingersnaps until finely ground. Transfer to a bowl. Add the walnuts to the work bowl, process until finely ground, and add to the cookie crumbs. (Alternatively, for a nut-free crust, omit the walnuts and add an additional 4 oz/60 g of gingersnaps.) Add the sugar and 5 tablespoons melted butter and mix well, adding more butter if the crumbs don’t cling together. Transfer to a 9-inch (23-cm) springform pan and press over the bottom and 1½ inches (4 cm) up the sides (it is fine if the edges are uneven). Bake until lightly browned, 8–10 minutes. Let cool, then place in the freezer until ready to use.
Reduce the oven temperature to 300°F (150°C). To make the filling, in the clean food processor, combine the cream cheese, ricotta, sugar, egg yolks, cream, lemon zest and juice, vanilla, and salt and process until creamy, 4–5 minutes. Pour into a large bowl. In a separate bowl, using an electric mixer, beat the egg whites until stiff peaks form. Using a rubber spatula, gently fold the whites into the cheese mixture just until combined. Pour the mixture into the reserved chilled crust, smoothing the top.
Place the pan on a foil-lined baking sheet and bake for 30 minutes. Raise the oven temperature to 325°F (165°C) and bake until golden brown on top, the edges are firm, and the center still jiggles, 30–35 minutes. Turn off the oven, open the door, and leave the cake in the oven for about 3 hours (the center will fall slightly). Cover tightly with plastic wrap (do not let the wrap touch the surface) and refrigerate for at least 4 hours or up to 24 hours. To unmold, run a knife around the sides of the cake to loosen them from the pan, then remove the pan sides. If desired, dust the top with confectioners’ sugar. Serve cut into wedges, dipping a knife into hot water and wiping it dry before each cut.