Here is the recipe I promised:
Ingredients:
- ½ of a yellow onion
- olive oil
- 1 cup sourdough starter
- 3 cups dark rye flour
- 1 cup bread flour
- 1 tsp salt
- 2 tbsp molasses
- 1 cup warm water (98°F to 110°F)
First Day
Dice the onion and caramelize in olive oil on low heat in a cast iron pan with a lid - let cool (do not add salt.)
Dissolve the molasses into the warm water in a nice big Pyrex bowl. Add your starter. With a fork, slowly add 1½ cups of rye flour and the cooled onions. The consistency should be like muffin batter.
Cover with a tea towel and put on top of the refrigerator for 24 hours... really.
Second Day
Take the bowl off the top of the refrigerator and uncover. Notice that a nice brown film has formed on top - don't worry. Mix in 1½ cups of rye flour slowly with a fork. When it gets to thick to use the fork, use your hands (make sure they are clean.)
Cover a clean flat surface with bread flour and begin kneading. Add flour as you go. You will need to knead for about 15 min to a half hour - you might want a friend to take over at some point. The dough will become elastic. If you press in you finger it will reshape itself.
Wash and dry the Pyrex bowl. Coat the bowl with oil. Place the loaf in the bowl, turn to coat with oil, cover, and put back on top the 'fridge. Let rise for 12 - 24 hours.
Third Day
Take down, uncover, clean hands, clean surface - knead again a little bit, with a little bit of flour and a sprinkle of salt. Form into a loaf shape and place on a pizza stone - I sprinkled some oat bran and pepper on the stone first. Score the top of the loaf with some slashes - this helps it to rise. Make sure your Pyrex bowl is still oiled - flip it over to cover the bread loaf. Place on top the 'fridge and let rise until 2x the size ~ 2-4 hours depending on how warm your place is.
Place a pie pan filled with water into the bottom of the oven. Preheat the oven to 350°F. Remove the bowl from the bread. Sprinkle some oats on top of the loaf and press in lightly.
Bake for about an hour. The bread will have a nice hollow sound when you thump it and the crust will be a toasty brown.

Let sit for 20 minutes then dig in! Serve with cold butter.