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1
Make sponge.
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2
Combine yeast and water in the bowl of a stand mixer and stir to dissolve.
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3
Whisk in flour.
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4
Cover with plastic wrap and let rise in a warm place until bubbly and doubled in volume, about 45 minutes.
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5
While starter is proofing, steam potatoes above 1 inch of boiling water until tender, 15 to 20 minutes.
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6
Mash with a potato masher or put through a potato ricer or sieve.
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7
Set aside to cool.
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8
Make dough.
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9
Whisk together yeast and water in a small bowl and let stand until creamy, a few minutes.
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10
Add to sponge in mixer bowl, along with the olive oil.
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11
Add mashed potatoes, flours (using smaller amount of unbleached flour) and salt and mix in with paddle attachment for 1 to 2 minutes, until ingredients are amalgamated.
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12
Change to dough hook and knead on medium speed for 8 to 10 minutes, adding more white flour if dough seems impossibly sticky (it will be sticky no matter what).
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13
The dough should come together and slap against the sides of the bowl.
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14
It will be tacky.
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15
Cover bowl tightly with plastic wrap and let dough rise in a warm spot until doubled, about 1 1/2 hours.
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16
Shape the focaccia.
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17
Coat a 12-x 17-inch sheet pan (sides and bottom) with olive oil.
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18
Line with parchment and flip the parchment over so exposed side is oiled.
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19
Turn dough onto the baking sheet.
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20
Oil or moisten your hands, as dough is sticky, and press out dough until it just about covers the bottom of the pan.
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21
Cover with a towel and allow it to relax for 10 minutes, then continue to press it out until it reaches the edges of the pan.
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22
Cover with a damp towel and let rise in a warm spot for 45 minutes to an hour, or until dough is full of air bubbles.
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23
Preheat oven to 425 degrees after 30 minutes of rising (30 minutes before you wish to bake), preferably with a baking stone in it.
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24
Place olive oil, sage and garlic in a small saucepan and heat over medium heat until the ingredients begin to sizzle in the oil.
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25
Allow to sizzle for 30 seconds, then remove from heat, swirl the oil in pan and transfer to a measuring cup or small bowl or ramekin.
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26
Allow to cool.
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27
Cut away the tough stems bottoms from the mushrooms and tear large mushrooms into smaller pieces.
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28
In a large bowl, toss with salt and pepper and the cooled olive oil mix.
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29
With lightly oiled fingertips or with your knuckles, dimple the dough, pressing down hard so you leave indentations.
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30
Arrange the mushrooms over the dough.
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31
Drizzle on any oil left in the bowl.
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32
Place pan in oven on baking stone.
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33
Spray oven with water 3 times during the first 10 minutes of baking, and bake 20 to 25 minutes, until edges are crisp and the top is golden.
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34
If you wish, remove the focaccia from the pan and bake directly on the stone during the last 10 minutes.
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35
Remove from oven, remove from pan at once and cool on a rack.
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36
Arrange the whole sage leaves over the top.
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37
If you want a softer focaccia, cover with a towel when you remove it from the oven.
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38
Serve warm or at room temperature.
