48 hours before beginning rye bread, make this starter: Mix 1 T. dry yeast in 2 cups of tepid water.
Beat in 2 cups of rye flour. Add a small onion, peeled and halved. Cover the bowl with plastic wrap. Let stand at room temperature for 24 hours.
Remove onion. Beat in 1 cup tepid water and 1.5 cups rye flour. Cover and let stand for 24 hours longer.
This can be used immediately or refrigerated for 24 hours. This preparation makes about 4 cups of starter (a bit more than required for the bread).
Combine 1/2 cup warm water, yeast, sugar, and let stand until double (10 min.).
Dissolve salt in remaining water. Mix in sourdough starter, then yeast mix.
Add gluten flour and 2 cups of all-purpose flour and optional caraway seeds; make a soft dough.
Spread 1.5 cups flour on kneading surface and turn dough out on it. Knead, adding more flour, to make a soft dough. Do not over-knead. The dough should be only slightly elastic, even a bit sticky. Form dough into a ball, and put in an un-greased bowl. Cover with plastic, and let rise until double (1.5 hours).
Knead, cover with towel, and let rest for 15 minutes. Divide into 2 parts. Form each into 12 inch loaf. Pinch seam, and place seam down on cornmeal-dusted sheet. Cover and let rise until 3/4 proof.
Put a large pan with 2 inches water in oven. Preheat to 400 °. Place quarry tiles on upper shelf of oven. Brush loaves with egg-white glaze, slash with knife. Sprinkle seeds on top. Bake for 30 minutes on tiles. Brush again with glaze; bake an additional 20 to 30 minutes.