Dissolve the yeast and sugar in lukewarm water (105 degrees F.). (Lukewarm water feels neither hot nor cold when a drop is placed on your wrist.) Let sit for 10 minutes to proof the yeast. If there are no bubbles in the water after this time, the yeast is inactive and you should start over with new yeast.
Add the salt (optional), oil, eggs and most of the flour to the 2 1/4 cups of water. Mix. Place dough on floured board and knead for 5-10 minutes.
Place dough in a clean bowl, brush some oil on the top (optional) and cover the bowl with a towel. Let rise in a warm place until doubled, about 1 1/2 hours.
Punch the dough down and let rise until doubled again (maybe another hour).
Punch dough down and place onto floured board. Form into two loaves. You can make normal loaves, or you can cut the dough and braid (with three or six pieces per loaf), or you can make braided or un-braided rolls, or you can roll the dough out until it's about 1/4 inch thick and then spread fried onions over it. Place loaves into oiled pans.
Cover pans and let rise for about 30 minutes. (Skip this step if you've rolled the dough out and covered with onions.)
Mix the egg yolk with about 1 tablespoon of water. Brush loaves with this mixture. (Again, skip this if you've rolled the dough out.) Bake at 400 degrees F. for 20 minutes, then lower heat to 350 degrees F. and bake for 40 additional minutes. If you are making rolls, you probably need to bake for a total of 20-25 minutes.
You can add poppy or sesame seeds to the dough, or you can sprinkle the seeds on top. This bread is traditionally made with white flour, but is still good if made with all or part whole wheat flour. I think its good taste comes from the oil and eggs in the dough.
NOTES: Challah is a traditional Jewish bread that is part of many festivals, including Sabbath dinners and weddings. For making a wedding loaf, this recipe can be doubled and used to make one braided loaf. It will fill up the entire oven, and you will have to find a cookie sheet big enough to put the loaf on. Yield: 2 loaves.