Step 1 Dissolve the yeast and sugar in one-fourth cup warm water (105 to 115 degrees) and set it in a warm place.
Step 2 If the egg is cold from the refrigerator, cover it with hot water and let it sit a few minutes to bring it up to room temperature. Combine 1 cup of the flour and the salt in a bowl and make a well. Break the egg into the middle of it; add the creme fraiche and pour in the yeast mixture, which should be foamy with bubbles. Mix everything together with a wooden spoon to form a smooth, soft dough, adding more flour as necessary. Dust it with flour, gather it into a ball, set it in a clean bowl and cover. Let the dough rise in a warm place, 45 minutes to an hour. If you are not ready to shape the dough at this time, punch it down and let it rise again.
Step 3Flatten the dough, place it in the center of the tart pan, and press it out to the edge using either your knuckles or the heel of your hand. Add only enough flour to keep the dough from sticking. If the dough shrinks back while you are shaping it, cover it with a towel, let it relax for 20 minutes, then finish pressing it out. It should be about one-fourth inch higher than the rim of the pan. It can be filled immediately or refrigerated until needed.