I have not made a porridge-style bread in a long time. The porridge was made with oats, malted wheat flakes, and rye chops. Instead of water, I used half-and-half, which adds a richer flavor.
Durum, rye, and Stardust wheat were all milled with my Mockmill 200. The rye was used whole, and the durum and Stardust were sifted with a #30 drum sieve and then re-milled at the finest setting with about 3% of the bran removed.
I baked the potatoes and removed the skins before pulsing them in my mini-food processor.
I used my Ankarsrum to mix up the dough and open-baked it on a baking stone with steam.
I was very happy with the flavor and overall bake. It’s kind of like a challah on steroids 
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Formula
Levain Directions
Mix all the levain ingredients together for about 1 minute and cover with plastic wrap.
Let it sit at room temperature for around 6-7 hours or until the starter has almost doubled. Either use in the main dough immediately or refrigerate for a few hours before using.
Porridge Directions
Place the grains in a small pot and slowly add about half of the half-and-half. Let it cook on low heat, stirring every minute or so. Add more half-and-half as the mixture heats up and thickens. The porridge is done once the liquid is absorbed. Remove it from the heat and let it cool before using it in the main dough. You can make this ahead of time and refrigerate it, or place it in the refrigerator or even the freezer to cool it down before using.
Main Dough Procedure
Note: I use an Ankarsrum Mixer, so my mixing order is slightly different from that of a KitchenAid or other mixer. Add all the water to your mixing bowl except for about 1/4 of the water. Add all the flour to the bowl and mix on low for 1 minute, until it forms a shaggy mass. Cover the mixing bowl and let it rest for 20 minutes. Next, add the levain, egg yolks, cooled porridge, and half of the remaining water, and mix on medium-low speed (about speed 4) for around 15 minutes, until you have a nicely developed dough. Let the dough rest for 20-25 minutes. Mix on speed 3-4, adding more of the remaining water as needed. Mix for 12 minutes and then add the potatoes and mix until incorporated.
Remove the dough from the bowl and place it in a lightly oiled bowl, and do several stretch and folds. Make sure the dough is as flat as possible in your bowl/container, and measure the dough in millimeters and take the temperature of the dough. Based on the chart from http://www.thesourdoughjourney.com, determine what % rise you need and make a note. If you have a proofer, decide what temperature you want to set it at and what rise you are aiming for.
Proof for 30 minutes at the required temperature and then do several sets of coil-folds to further develop the gluten. Let it proof another 30 minutes and do additional coil folds. After another 30 minutes, do 1 more set of coil folds and let the dough finish bulk rising.
Once the dough reaches the desired bulk rise, pre-shape and let rest for 20 minutes. Finish shaping and place in your banneton, bowl, or on your sheet pan, and cover it with a moist tea towel or plastic wrap sprayed with cooking spray. Let it sleep overnight in your refrigerator. If you desire, you can leave the dough uncovered if it’s in a banneton.
When you are ready to bake, preheat your oven to 500°F an hour beforehand and prepare for steam. When ready to bake, score as desired. Prepare your oven for steam. I have a heavy-duty baking pan on the bottom rack of my oven with 1 baking stone above the pan and one on the top shelf. I pour 1 cup of boiling water into the pan right after I place the dough in the oven. I then lower the temperature of the oven to 450°F. Bake until they are nice and brown, and the internal temperature is at least 205 – 210°F.
Take the bread(s) out of the oven when done and let them cool on a baker’s rack for as long as you can resist.
