Gather the ingredients.

Mix the flours, oats, salt, caraway seed, and instant yeast together in a large bowl.

In a small bowl, mix together the milk and vinegar to create a sour milk.

Add the sour milk , yogurt, and sourdough starter to the dry ingredients and begin to mix with a large spoon. If you have access to a standing mixer , put in all ingredients and mix at low speed with the help of the hook attachment.

Mix all ingredients until the dough forms a ball, adding 1 to 2 tablespoons of water if needed. The dough should be slightly sticky.

Continue kneading either with the mixer or on a lightly floured board for 5 to 7 minutes.

Let the dough rest for 5 minutes, then knead again for 1 minute.

Form dough 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 in size.

Turn out the dough onto a lightly floured board and pat it into a rectangle. Indent with fingertips down the middle.

Fold a third of the dough to the middle, lengthwise, pulling the dough taut on the bottom. Press the seam a little to seal.

Fold the other third to the middle, pulling the dough taut, and pinch the seam closed.

Roll over, the dough seam-side down, and rock gently while rounding the ends to make the loaf either longer or fatter, whichever you prefer.

Place dough on parchment paper, baking sheet, or silicone mat, then dust the top with flour, and let it rise until doubled.

Heat the oven to 500 F for 1 hour before baking. About 30 minutes before you plan on baking, slash the top with a sharp razor blade or lame at least ¼-inch deep.

Place a baking stone inside of the oven while it preheats, following the manufacturer's instructions. Place an old pan on the bottom rack and set the second rack in the middle.

Once the oven is hot, place the bread on the middle rack, still on the parchment or floured baking pan, or transfer it onto the baking stone. Immediately, pour 2 cups of boiling water into the bottom pan, and close the door.

Using a spray bottle filled with water, spray the sides of the oven after 2, 5, and 7 minutes. Next, turn the oven temperature down to 450 F and bake for 20 minutes. Turn the oven temperature down again to 350 F and bake for 20 to 30 minutes, or until an instant-read thermometer measures 190 F to 200 F or the loaf is brown and sounds hollow when firmly tapped on the bottom.

Remove and let cool 2 hours before slicing.