For the crust:
1. Preheat oven to 180°C/ gas mark 4.
2. Tightly wrap the outside of a 22cm diameter springform pan with 7cm-high sides with 3 layers of heavy-duty foil. Finely grind the almonds, biscuit crumbs, and sugar in a food processor. Add the butter and process until moist crumbs form. Press the almond mixture onto the bottom of the prepared pan (not on the sides of the pan). Bake the crust until it is set and beginning to brown, about 12 minutes. Cool. Decrease the oven temperature to 160°C/ gas mark 3.
For the filling:
1. Using an electric mixer, beat the cream cheese, mascarpone cheese, and sugar in a large bowl until smooth, occasionally scraping down the sides of the bowl with a rubber spatula. Beat in the lemon juice and vanilla. Add the eggs, one at a time, beating just until blended after each addition.
2. Pour the cheese mixture over the crust in the pan. Place the springform pan in a large roasting pan. Pour enough hot water into the roasting pan to come halfway up the sides of the springform pan. Bake until the center of the cheesecake moves slightly when the pan is gently shaken, about 1 hour 5 minutes (the cake will become firm when it is cold). Transfer the cake to a rack; cool for 1 hour. Refrigerate until the cheesecake is cold, at least 8 hours and up to 2 days.
For the topping:
1. Combine the chocolate-hazelnut spread and cream in a small bowl. Heat in the microwave until warm, stirring every 20 seconds to blend, about 1 minute.
2. To serve, cut the cake into wedges. Drizzle the chocolate sauce over the wedges.