As an exception to most baking, make sure to use ingredients as cold as possible when making pastry: so cubed butter straight from the fridge and ice cold water. Keeping pastry cold helps avoid overworking the gluten and keep it relaxed to avoid shrinkage during baking.
Handle the pastry dough as little as possible throughout making. This is especially important when bringing it together, as overworking the gluten will lead to tough pastry.
When placing the pastry into the tin, try not to stretch the dough to make it fit. Stretching the dough may look fine once in the tin, but doing so will result in the dough shrinking back to its pre-stretched size and shape during baking, spoiling the look of the tart.
Chill the pastry again once rolled and shaped into the tin. Chilling the pastry a second time allows the gluten to relax again, helping it hold its shape better and not be tough once baked.
Giving the baked pastry a light egg wash seals the pastry further, helping to ensure the filling does not soak into the pastry, resulting in either a soggy pastry or cracks in the lemon filling.
Fill the pastry to near the top with lemon filling. Pour as close to the pastry as possible to avoid any air bubbles getting caught in the custard.
The lemon tart is baked when the middle of the filling still has a slight wobble. Remove from the oven but leave the tart in the tin to cool completely and firm up.
Once cool, store the lemon tart in the fridge undecorated. decorate the tart as you wish just before serving.