Preheat the oven to 175°C. In a medium bowl, combine the flour, 3 tablespoons sugar and a pinch of salt. Add the butter and lemon zest and work into the flour mixture until crumbly using your fingers or a pastry cutter. Separate the whole egg, adding the white to the other two for the meringue topping. Add the yolk to the dough and mix in until a soft dough forms. Press into the bottom and up the sides of a 9-inch pie plate. Bake until golden, about 25 minutes. Cool. Lightly score the pie shell with a sharp knife (cutting about halfway through the crust) into 10 slices to make cutting the frozen pie easier.
Place the lemon ice cream in a large bowl and mix with a stiff spatula or wooden spoon until smooth, about 2 minutes. Mix in the food colouring if using. Spoon into the pie shell, smooth the top and place in the freezer until firm, 2 hours or overnight.
Preheat the oven to 260°C. Place all 3 egg whites, the remaining sugar and a pinch of salt in a large heatproof bowl and put it over a pot with 1-inch of simmering water. Whisk until the sugar is dissolved, the eggs are warm and the mixture is fluffy. Whip with an electric mixer on medium-high speed until cooled, thick, glossy and stiff peaks form, about 2 minutes. Beat in the vanilla extract until just combined. Spread over the frozen pie and place in the oven until the meringue just browns, 1 to 2 minutes. Slice and serve.