01. In a large mixing bowl, combine the all-purpose flour, shortening, and butter. Use your fingertips to rub the fat into the flour until the mixture resembles breadcrumbs. Add the egg, apple cider vinegar, lemon zest, and salt to the mixture. Gradually add water, mixing until a dough forms. Wrap the dough in plastic wrap and chill in the refrigerator for about 30 minutes.

02. Preheat your air fryer to 180°C (360°F). Roll out the chilled dough on a lightly floured surface until it is about 1/8 inch thick. Carefully line a tart tin with the rolled-out dough, pressing it into the edges and trimming any excess. Prick the base with a fork to prevent it from puffing up during baking.

03. Place a sheet of parchment paper over the pastry and fill it with baking beans or rice. Bake in the preheated air fryer for 12-15 minutes until the edges are lightly golden. Remove the parchment paper and baking beans/rice, and bake for an additional 5 minutes to ensure the base is cooked through.

04. In a saucepan, heat the whole milk and heavy cream over medium heat until just simmering. Do not let it boil. In a separate bowl, whisk together the egg yolks, caster sugar, and vanilla extract until well combined. Slowly pour the hot milk and cream mixture into the egg mixture, whisking continuously to prevent the eggs from scrambling.

05. Pour the prepared custard filling into the pre-baked tart shell. Grate fresh nutmeg over the top of the custard for added flavor.

06. Lower the air fryer temperature to 140°C (280°F). Bake the custard tart in the air fryer for 35-45 minutes, or until the custard is set but still slightly wobbly in the center. Remove the tart from the air fryer and let it cool completely before serving.