Oat Porridge Boule with Cheddar and Parmesan

Steps

1. Make the porridge first: in a small pot, simmer the oats with the water for 4 to 5 minutes, stirring, until thick; spread on a plate and cool to lukewarm, about 20 minutes.
2. Meanwhile, cube the cheddar, slice the green onion, and beat the egg; chill the cheddar and egg until needed.
3. In a large bowl, whisk the flour, yeast, salt and half of the Parmesan.
4. Add the cooled oat porridge and the lukewarm milk and stir into a sticky dough; counting the water and the milk's water, it is about 85% hydration with 2% salt by bakers' percentages, softened by the oats soaking up liquid.
5. Knead on a lightly floured counter 8 to 10 minutes, adding the butter partway through, until smooth, tacky and elastic.
6. Flatten the dough, scatter the cheddar and green onion over it, roll it up and knead gently for 1 minute to spread them evenly.
7. Place in a lightly floured bowl, cover and rise at about 75°F (24°C) for 1 1/2 hours, until doubled in size.
8. Shape into a tight round boule and set it seam-side down on a sheet of parchment; cover and proof at about 75°F (24°C) for 45 to 60 minutes, until puffy and it springs back slowly when poked.
9. During the proof, place a lidded Dutch oven in the oven and preheat to 425°F (220°C) for at least 30 minutes.
10. Brush the boule with the beaten egg, sprinkle with the remaining half of the Parmesan, and score a shallow cross on top.
11. Lift the boule by the parchment into the hot Dutch oven, cover and bake 25 minutes.
12. Uncover and bake 15 to 20 minutes more, until deep golden and glossy and the center reaches 190°F (88°C).
13. Cool the loaf on a rack for at least 1 hour before cutting into 12 slices.