Preheat the oven to 350˚F.
Place the Graham crackers in bowl of a food processor. Pulse until the crackers are broken down into fine crumbs. Transfer to a medium sized bowl.
Add melted butter to the ground crackers and use a spoon to mix until everything comes together.
Press the Graham mixture into a 9-inch removable bottom tart pan, pressing onto the sides and the bottom. Use a straight sided measuring cup to help you pack in the crust tightly. Place the tart pan on a baking sheet.
Place the baking sheet on the middle shelf in the oven and bake for 10 minutes. Remove from the oven and let it cool completely.
To make the filling, in a large bowl combine condensed milk, heavy cream, lemon juice, zest, egg yolks, and salt. Strain the mixture and pour the filling into the tart shell.
Place the tart on a baking sheet, put it on the middle rack, and bake at 350˚F for 40 minutes until the edges of the tart are set and middle is still wobbly. Let the tart cool to room temperature then refrigerate it for 4 hours for the filling to set.
To make the meringue, combine sugar and water in a small saucepan over medium-high heat. Bring to a boil and cook until the it becomes syrupy and the temperature registers 240˚F on the candy thermometer, about 5 minutes.
In a bowl of a stand mixer with whisk attachment, whip the egg whites and cream of tartar on high speed to soft peaks. While you're whipping the eggs, carefully pour in the hot syrup down the side of the bowl in a steady stream.
Once all the syrup has been added, continue to whisk on high speed for another 5 minutes until the mixture is glossy and stiff peaks form.
Spoon the meringue on top of the lemon tart and use a blowtorch to gently brown it. If you don't have a torch, place the tart in your oven and broil it for 30-45 seconds until the peaks are browned, keeping a close eye on it so it doesn't burn.
When ready to serve, slice the tart in eight and transfer each slice to a serving plate.