In a bowl dissolve yeast and sugar in the water then move it to a warm place while you gather the other ingredients.
If the egg is cold warm it in a bowl of hot water.
Combine the 1 cup of the flour, salt, and lemon zest in a bowl and make a well in the middle.
Break the egg into the middle of the well; add the softened butter; and pour in the yeast mixture which should be foamy with bubbles.
Mix with a wooden spoon to form a soft smooth dough. Dust it with flour and gather into a ball. Set it in a clean bowl and cover with cling film or a kitchen cloth.
Let the dough rise in a warm place until it has doubled in bulk (about 45 minutes to an hour).
Flatten the dough place it in the centre of a 10 inch tart ring and using either your knuckles or the heel of your hand press it out to the edge of the ring. Add only enough flour to keep it from sticking. If the dough shrinks back while you are shaping it cover it with a towel and leave it to relax for 20 minutes before you finish pressing it out.
It should be thin on the bottom and thicker at the sides, about 1/4 inch above the rim of the pan.
It can be filled immediately or refrigerated until needed. Once the tart is filled, bake in the middle of a 375 degree oven.