1. Preheat oven to 350°F/177°C.  Grease and line an 8-inch springform pan with parchment paper. Tightly wrap the outside of the pan (both sides and bottom) with aluminum foil. Set aside.

For the crust

1. In a medium bowl, mix graham cracker cookie crumbs, sugar, salt and melted butter until combined. Press the crumb mixture evenly into the bottom of the prepared pan. Bake for 8 minutes. Allow to cool while preparing the filling.

For the filling

1. In a large bowl, using a hand-held mixer or stand mixer fitted with the paddle attachment, beat together the cream cheese, mascarpone, and sugar until smooth and creamy, about 2 minutes. On medium-low speed beat in eggs one at a time, allowing each egg to fully incorporate before adding the next. Scrape down the mixing bowl between each addition. Add the vanilla and mix until smooth. Add the salt, lemon juice and lemon zest and mix until just combined.
2. Place the pan with the crust into a larger deep baking pan. Pour the filling into the crust. Pour boiling water into the larger baking pan until it is halfway up the sides of the springform pan.
3. Bake for 60-70 minutes or until the edges are set and the center is almost set. The center of the cheesecake will jiggle slightly when you gently shake the pan. Turn off the oven and prop open the oven door slightly. Allow the cheesecake to sit in the cooling oven for 1 hour while the water bath cools down.
4. Carefully remove the cheesecake from the oven and from the water bath and allow to cool to room temperate. Once completely cooled, place in refrigerator for at least 4 hours or overnight.

For the meringue topping

1. Place egg whites, sugar, and salt in a heatproof bowl of a stand mixer. Set bowl over a small saucepan filled with an inch of simmering water. Whisk constantly until sugar is dissolved and egg whites are warm to the touch, about 5 minutes.
2. Transfer the mixing bowl to a stand mixer fitted with a whisk attachment.  Beat on medium-high speed until stiff, glossy peaks form, about 8-10 minutes.
3. Remove the rim of the springform pan and the parchment paper from the cheesecake.  Spread the meringue over the top of the cheesecake using the back of a metal spoon or an offset spatula.  Using a kitchen blow torch, carefully toast the meringue.
4. Keep the cheesecake chilled until ready to serve.