Rosemary Oat Porridge Rye Sourdough Boule

Steps

1. Feed the sourdough starter the morning of mixing; keep it at about 75°F (24°C) and use it at its peak, bubbly and doubled, 4 to 6 hours after feeding.
2. Make the porridge: simmer 80 g of the oats (the larger portion) with the whole milk in a small pot, stirring, for 4 to 5 minutes until thick and creamy. Stir in the rosemary and cool to room temperature, about 30 minutes. Set the remaining 20 g of oats aside for the top.
3. In a large bowl, mix the rye flour, the bread flour and the lukewarm water until no dry patches remain. Rest 30 minutes.
4. Add the sourdough starter, the molasses, the salt, the ground flaxseed, the sesame seeds and the cooled oat porridge. Squeeze and fold with a wet hand until evenly combined; the dough will be sticky and paste-like, as rye dough is. This is about 60% hydration from the water alone (about 95% counting the milk) and 2% salt.
5. Bulk rise at about 75°F (24°C) for 4 to 5 hours, giving 3 sets of gentle stretch-and-folds with wet hands, 30 minutes apart, in the first 1.5 hours. It is ready when puffed by about half with bubbles on top.
6. Shape: with wet hands, tip the dough onto a damp work surface, round it gently into a boule and roll the top in the reserved 20 g of oats. Place it oat side down in a bowl lined with parchment.
7. Cover and proof in the fridge overnight, 10 to 14 hours, until slightly puffed and it springs back slowly when poked.
8. Put a Dutch oven with its lid in the oven and preheat to 450°F (230°C) for 45 minutes.
9. Turn the dough out onto parchment, oat side up, score a shallow cross and lower it on the parchment into the hot Dutch oven. Bake covered 25 minutes, then uncover, reduce to 425°F (220°C) and bake 25 to 30 minutes more, until the center reaches 205°F (96°C).
10. Brush the hot crust all over with the melted butter, then loosely cover the loaf with a clean towel; this keeps the crust soft.
11. Cool on a rack at least 3 hours before slicing into 12 slices; rye needs the time to set.