Preheat the oven to 325F.

In a medium bowl, mix the matzah and graham cracker crumbs, sugar, and fleur de sel with a fork until combined.

Add melted butter and mix with the fork until the mixture is uniform, dark brown, and resembles the texture of wet sand.

Using the bottom of a water glass, press the mixture into a 9-inch pie plate, spreading the crust evenly along the bottom and up the sides of the pan.

Bake on the bottom rack for 10 minutes. Remove and let the crust cool completely.

Increase oven temperature to 350F.

In a mixing bowl, use a hand-mixer on medium speed to beat egg yolks and sweetened condensed milk until thick and creamy for about 2-3 minutes.

Add Key lime juice and zest, and mix until smooth for about 1 minute.

Pour the filling into the cooled crust and smooth the top. Bake on the middle rack for 12 minutes, or until the filling is just set. Let cool at room temperature for 30 minutes, then place in the refrigerator to chill for 4 hours before serving. 

Make the meringue topping: Using the mixer, beat egg whites with ½ cup sugar until still peaks form. Spread the meringue over the chilled pie. 

Set the oven to broil. Place the pie under the broiler for 1-2 minutes, watching closely, until the meringue is golden brown. Alternatively, you can use a kitchen torch to toast the meringue until golden.

The pie can be stored in an airtight container in the refrigerator for 3-4 days.