Scald milk, add the sugar, salt and oil. Stir until sugar dissolves, then let cool to lukewarm. Dissolve the yeast in the lukewarm water. Stir in the cooled milk mixture. Add white flour and mix well. Stir in caraway, onion and 2 cups of rye flour. Turn out onto rye floured board and knead until smooth, adding additional rye flour until dough is stiff. Grease a steep sided bowl. Shape dough into ball, and place in bowl, then turn upside down so that oiled, smooth side is on top. Cover with a towel and allow to double for 1 to 2 hours. Punch down dough, cover it and allow it to double again. Grease 3 loaf pans and sprinkle with cornmeal. Punch down dough again, then divide it into three. Let dough rest 10-15 minutes Shape into three loaves, place in pans, and sprinkle with kosher salt. Cover and allow to double again, about 1 hour Preheat oven to 500 degrees F for 1 hour. Throw 1/3 cup of water onto walls of oven before putting loaf in. (Omit for a less crusty bread). Bake 45 minutes at 450 degrees F, throwing water on the oven walls three times more in the first five minutes, or, alternatively, bake one hour at 350 degrees F, until it sounds hollow when tapped. Remove and allow to cool on rack.