Preheat the oven to 325°F. Line the bottom of a 9-inch springform pan with a parchment round.
To make the crust: In a food processor, combine the hazelnuts, sugar and salt. Pulse until the nuts are finely ground. Add the butter and pulse until moist clumps form. Press the crust on the bottom and about 1¼-inches up the sides of the prepared pan. Refrigerate until firm, about 20 minutes.
To make the filling: In a food processor, blend the cream cheese until smooth. Add the ricotta and process until well blended. Add the sugar, lemon zest, cornstarch, vanilla, and salt and process until smooth. Mix in the cream. Add the eggs one at a time, processing until smooth after each.
Wrap the outside of the crust lined pan tightly with 3 layers of heavy-duty foil, covering the sides completely. Pour the filling into the pan. Place the springform pan in a large baking pan. Add enough hot water to the baking pan to come halfway up the sides of the springform pan.
Place the cake in the baking pan in the oven. Bake until the edges of the cake are set and the center jiggles when the pan is gently shaken, about 1 hour 20 minutes. Remove the cake pan from the baking pan and place on a wire. Cool 1 hour. Remove the foil and refrigerate the cake uncovered overnight.
To make the sauce: In a heavy medium saucepan, combine 3 cups of the fresh berries (or 2½ cups of the frozen berries) and the maple syrup. Simmer until the berries begin to soften, about 3 minutes. Dissolve the cornstarch in the water and add to the saucepan. Simmer until the liquid thickens slightly, stirring constantly. Remove from the heat and immediately add the remaining 1½ cups fresh berries (or 1¼ cups wild berries), lemon zest, and nutmeg. Stir to combine. Cool 20 minutes. Cover and refrigerate until chilled.
Using a small, sharp knife, cut around the sides of the cheesecake to loosen. Remove the pan sides. Transfer the cake to a platter. Cut the cake into wedges and place on plates. Spoon some blueberry sauce over and serve.