For the filling: Preheat the oven to 400 degrees F.
Bring the sugars, cream of tartar and 2 cups water to a boil in a saucepan over medium-high heat, whisking thoroughly as the sugar dissolves. Add the butter, cinnamon, nutmeg, lemon zest and lemon juice and whisk until the butter is melted. Boil for 5 minutes. Remove from the heat and stir in the vanilla.
Put the crackers in a heatproof bowl and pour the syrup over the top. Let mixture sit for 5 minutes, stirring occasionally. Pour into the pie shell.
For the topping: Mix the crushed crackers, brown sugar, melted butter and cinnamon in a small bowl. Sprinkle over the top of the pie. Set the pie on a rimmed baking sheet and bake for 10 minutes, then lower the temperature to 350 degrees F and continue to bake until the top is golden brown and the filling is set, 18 to 20 minutes. Allow the pie to cool slightly.
For the faux whipped cream: Whip the chickpea liquid with an electric mixer until bubbles begin to form. Add the cream of tartar and continue to whip on high speed until peaks form, 2 to 3 minutes. Pour in the vanilla and continue whipping for another 1 to 2 minutes. Slowly add the confectioners’ sugar while whipping an additional 3 to 5 minutes.
Dollop slices of the pie with the faux whipped cream and serve immediately (see Cook’s Note).