This sour dough makes bread with an excellent punchy flavor and a good bitey crust.
Combine yogurt, sugar, and salt in a large heatproof bowl. Pour in the boiling water and stir well. When mixture is tepid, sprinkle on the yeast and allow the mixture to sit for 10 minutes, or until the yeast is completely dissolved. Beat well, then start stirring in the flour. Continue to add flour until dough is very stiff, then turn out onto a floured board and knead, adding more flour as necessary, until you have a smooth, shiny dough that is not sticky. Return to bowl, cover loosely with a barely damp tea towel, and permit to rise until fully doubled. Dough may be formed into loaves at this point, but the flavor will be improved if dough is punched down and permitted to rise again at least once, ideally twice, before shaping and baking. (Each successive rising will take less time.)
Punch down dough and shape into 4 long loaves. Place each in a cornmeal-sprinkled pan and allow to rise, uncovered. Preheat oven to 400 degrees F. Slash risen loaves with a razor blade and place them in oven. Bake 15 minutes, lower heat to 350 degrees F, and bake 40 minutes longer or until loaves are nicely browned and sound hollow when tapped. About 5 minutes before you expect them to be done, brush loaves with glaze. When done, turn out of pans immediately and cool on wire racks.