1DOUGH: Mix flours, oats, salt, caraway and instant yeast together in a large bowl. Add the sour milk, yogurt and sourdough culture and begin mixing, either with an electric mixer or by hand. Mix until dough forms a ball, adding water if needed. The dough should be slightly sticky.
2Continue kneading, either with the mixer or on a lightly floured board for 5-7 minutes. Let the dough rest for 5 minutes, then knead again for 1 minute. Form into a smooth ball and place in an oiled bowl, turning to coat the top. Cover with a clean dish towel and let rise in a warm place until doubled.
3Turn out onto a lightly floured board and form into a typical long loaf as follows:
Degas as little as possible.
Pat dough into rectangle.
Indent with fingertips down the middle.
Fold one third to middle, lengthwise, pulling dough taut on the bottom.
Press seam a little to seal.
Fold other third to middle (pulling dough taut) and pinch seam closed.
Roll over, seam-side down and rock gently while rounding the ends and making loaf longer or fatter, as you wish.
4Place on parchment paper on a baking sheet or cardboard, dust top with flour and let rise until doubled. About 30 minutes before you plan on baking, slash the top with a sharp razor blade at least 1/4 inch deep.
5Heat oven to 500 degrees for 1 hour before baking. Use a baking stone if you have it, according to manufacturer's instructions. Otherwise, place an old pan on the bottom rack and set the second rack in the middle.
6Place the bread on the middle rack (still on the parchment or floured baking pan), pour 2 cups of hot water into the old pan and close the door.
7Using a spray bottle filled with water, spray the sides of the oven after two, five and seven minutes. Turn oven down to 450. Bake for 20 minutes.
8Turn oven down to 350 and bake for another 20 to 30 minutes, or until a temperature probe measures 190-200F or loaf is brown and sounds hollow when tapped. Remove and let cool 2 hours before slicing.
9NOTE: You can purchase sourdough culture online. King Arthur Sourdough Starter, Goldrush Sourdough Starter, or Ed Wood's International Starters can all be purchased online. Or you can obtain a culture sample from a friend or start your own.