In a large mixing bowl, cream softened butter with powdered sugar until smooth and fluffy. Add the egg, almond meal, and any optional zest; mix well. Gradually incorporate flour and salt until a smooth dough forms, then chill for at least 1 hour.

Roll out the chilled dough to fit your tart pan and carefully line the pan with it. Dock the base with a fork to prevent bubbling, then freeze for 30 minutes. Blind bake at 160°C (325°F) for 25-30 minutes until golden brown. Let it cool completely.

Zest and juice your oranges. In a saucepan, heat the juice over medium heat until it simmers. In a separate bowl, whisk together eggs, caster sugar, and cornstarch. Slowly temper in the hot juice, stirring continuously. Return to the saucepan and cook until thickened, stirring constantly.

If using gelatine, hydrate it in a small bowl, then whisk into the hot cream mixture. Stir in butter and additional orange zest until well combined. Allow the cream to cool slightly before pouring it into the baked tart shell, smoothing the top with a spatula.

Refrigerate the tart for at least 2-3 hours until the filling is set and firm. Once set, slice and serve chilled, garnished with your favorite toppings.