Prepare the jello by mixing it with the boiling water. Place it in the fridge to cool down completely.
Make the crust: mix 3 cups of the crushed graham crackers with the melted butter and press it into the bottom of a greased 9x13-inch baking dish.
Whip the heavy cream to stiff peaks and reserve.
In a large bowl, using a hand-held or stand mixer, beat together the cream cheese, lemon zest, lemon juice, sugar, vanilla extract, and salt. Pour in the cooled jello slowly and beat for one minute.
With the mixer on low, beat in the whipped cream until fully combined.
Spoon the cream cheese filling over the graham cracker crust and sprinkle the remaining ½ cup crushed crackers on top. Refrigerate at least 2 hrs or overnight before serving