Sage and caramelized onion sourdough bread

Levain/sponge (night before or at least 9hrs before baking)

1. Mix the levain/sponge ingredients together in a bowl, making sure they are well mixed. Cover the bowl with a cloth or cling wrap/film and leave in at room temperature overnight. Alternatively, leave for at least 6 hours at warm room temp, or around 8 hours more normal temp. You should see it noticeably have bubbles on top.
2. During this time, finely dice the onions. Warm the olive oil in a small skillet over a medium heat and add the onion, sugar and salt. Cook for at least 15-20 minutes until the onions soften and start to caramelize. Reduce the heat and/or add a little more oil as needed to avoid the onions burning. Once the onions are starting to caramelize, add the chopped sage and cook for a few more minutes. Remove from the heat and allow to cool. (Can make ahead and set aside.)

Making dough and initial rise

1. After the levain's rise, mix in the additional flours and water but not the salt. Leave the mixture to rest for around 30 minutes.  
2. Add the salt, mix in (you'll feel the dough tense up), then turn the mixture onto a lightly floured surface. Spread it out slightly and top with the onion mixture. Fold in the sides to help hold the onions in the loaf then knead the dough for a few minutes, pushing one side out with the heel of your hand, folding over, then turn 90 degrees and repeat. As you do so, you'll naturally work in the onion and distribute it evenly.
3. Lightly brush a large bowl with oil then form the bread into a ball and put in the bowl. Cover and leave to rise another 2 hours, opening it up to fold roughly every 30 minutes. For each fold, pull on one side of the dough, stretch it up pretty much as far as it will go and fold it over the rest of the dough. Turn the bowl 90 degrees and repeat. Make at least 4 of these stretches for each fold (I sometimes do 6 if it feels like it needs it). During this time, line a Dutch oven or other large baking dish with a lid with parchment.

Forming loaf and second rise/proof

1. After you have made three folds and let it have a further half hour (ie 2 hours total), turn the dough out onto a lightly floured surface. Fold the sides over tightly in to the middle to form a tight ball (bring in to middle in 4 or 5 places). Repeat this process one more time with floured hands, picking it up and turning over at the end to bring it in to a ball or oval shape. Place the dough, smooth side up, join side down, in the lined Dutch oven/pan.
2. Cover the pan with cling wrap/film and leave to rise approx 2 hours.

Baking the loaf

1. When ready to bake, remove the cling wrap/film then hold the corners of the parchment to lift the loaf carefully out of the Dutch oven/pan. Let it sit as you preheat the oven to 450F/230C with the Dutch oven/pan warming in the oven at the same time.
2. When the oven has heated, take out the Dutch oven/pan and carefully put the parchment with the loaf on into the pan. Score the loaf in the middle then cover with the lid and bake for around 20 minutes. Remove the lid and bake for another 20 minutes until the loaf is golden brown and sounds hollow to tap.
3. Transfer the loaf to a cooling rack and allow it to cool before slicing.