Make a day ahead. Pour water first in the mixer bowl. Add flour, salt and yeast. Mix on low to combine – increase speed to medium and mix for 8 min. Make a tight ball. Transfer in a lightly oiled bowl, cover and leave out for 2 hours and place in the refrigerator overnight. Use half of the starter for the recipe; save and freeze the remaining starter for later use.
Mushroom stock: bring half of the spring water (250g) to a boil and dissolve mushroom base and porcini powder. Remove from heat and add the remaining cold water; let cool to 60ºF/15ºC.
Meanwhile, heat up a frying pan, toast hazelnuts and crush. In the same pan, sautee mushrooms on high heat with a drizzle of olive oil for about 6 min. Transfer mushrooms in a large plate (drain if too wet), season with ground black pepper, add nuts and chill.
*If using dried mushroom soup, add up 30g water to the dough.
In a 7qt (7.5L) mixing boil, add mushroom stock, flours, salt and instant yeast – mix on low to combine. Increase speed to medium-high and mix for 6 to 8 min. Add starter and continue mixing for 5 min more and incorpotate the chilled mushroom-hazelnut mixture. Internal dough temperature should reach 75/79ºF(24/26ºC).
Cover and let ferment for an hour (called bulk or primary fermentation). Degas/deflate and let ferment for another hour (2 hours total). Transfer dough onto a floured work surface; gently deflate and fold into a fat pillow (seam side down). Let rest 5 min, flip dough (seam side up) and transfer on a floured baker’s couche (best for proffing bread) otherwise floured silicone mat. Cover and let proof for an hour.
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Preheat oven to 480ºF/250ºC with a ≈ 7qt/7L Dutch oven for 30 minute prior baking. Divide proofed bread in half – transfer one loaf onto another tray lined with silicone mat; cover and chill. Carefully, remove hot Dutch oven from the oven. Gently, grab the couche and slide bread upside down in the hot Dutch oven; put the lid back on and bake for 60 minutes. Bake remaining bread as is for an hour, or divided into 4 pieces straight over a large pizza stone for 35 minutes. Let cool to room temperature – Enjoy!