Make your levain: in a small glass mixing bowl or a large glass jar, thoroughly mix the sourdough starter, 30 g all purpose flour, 30 g whole wheat flour, and 60 g filtered water. Cover tightly and place on the counter for 12 hours.

At the same time, prepare your seed & oat soaker: place flax seeds, sunflower seeds, sesame seeds, rolled oats and boiling water in a medium glass bowl. Leave on the counter to sit while your levain ferments.

About an hour before you're expecting your levain to be ready, prepare the tangzhong: add 80g whole wheat flour and 325 g whole milk to a small saucepan. Heat it over medium-low heat, whisking constantly, until the mixture changes to a paste-like consistency. This will probably take 5-6 minutes, and it will seem like nothing is happening, until it does. 

Remove the tangzhong from the heat. Scrape it all onto a plate and set it on the counter until it fully cools to room temperature.

Meanwhile, remove your butter from the fridge: you want this at a cool room temperature as well.

Check your levain for readiness: it should be risen, bubbly throughout, and have a mild sour aroma. If it doesn't show these signs check again in 30 minutes.

Get a large bowl- I prefer a stand mixer for this dough, fitted with the dough hook.

Add the water, remaining 100g whole wheat flour and 550 g all-purpose flour, honey, fine sea salt, the levain and the tangzhong to the bowl. Mix at low speed until all incorporated. If your dough is VERY dry add another splash of water, just enough to get it to come together.

Increase the mixer speed to medium for 5-6 minutes. The dough will likely still stick to the sides of the bowl. 

Decrease the mixer speed back to low and add the butter one pat at a time until it is all incorporated into the dough. Increase the speed back to medium for a few more minutes. The dough should start to pull away from the sides of the bowl.

Add the seed mixture to the bowl and mix on low speed until evenly distributed.

Cover the bowl and place on the counter for bulk fermentation.

Set a timer for a half hour and complete a thorough set of stretch and folds. Coil folds can work well for this dough. Repeat twice more at half hour intervals, then leave the dough to sit for the remainder of bulk fermentation, two and a half more hours to equal four hours total. 

At the end of bulk fermentation, assess your dough: it should be visibly risen in your container, "domed" at the sides (sloping down towards the bowl, instead of totally flat), and you may see scattered bubbles. If these signs aren't present, leave for 30 minutes and check again.

Use a bowl scraper to turn your dough out onto a clean work surface and use a bench scraper to divide it in two. Use lightly wet hands and your bench scraper to gently shape each half of the dough into a round. Let the pre-shaped dough rest on the counter for thirty minutes.

Meanwhile, butter or grease two loaf pans or small Pullman pans (my preference!)

Use your bench knife to tightly shape each half of the dough into an oblong loaf the length of your pan. Transfer each loaf into a pan. Cover the pans with plastic wrap and place in a warm place on the counter to proof.

Allow the dough to proof for 3-4 hours, until the dough is close to the top of the pan. It will feel jiggly and soft to the touch.

Towards the end of the proofing time preheat the oven to 425 degrees Farenheit. 

If you have any large bubbles at the top of your loaves, gently pop them before placing the loaves in the oven.

Bake your loaves for 20 minutes, then decrease the oven temperature to 350 and bake for another 20-30 minutes, until the tops of the loaves are golden brown. 

Allow your loaves to cool in the pan for 5-10 minutes, then remove them to a wire rack to cool completely before slicing.