POTATO FOCACCIA WITH OYSTER MUSHROOMS

Steps:

• Make sponge. Combine yeast and water in the bowl of a stand mixer and stir to dissolve. Whisk in flour. Cover with plastic wrap and let rise in a warm place until bubbly and doubled in volume, about 45 minutes.
• While starter is proofing, steam potatoes above 1 inch of boiling water until tender, 15 to 20 minutes. Mash with a potato masher or put through a potato ricer or sieve. Set aside to cool.
• Make dough. Whisk together yeast and water in a small bowl and let stand until creamy, a few minutes. Add to sponge in mixer bowl, along with the olive oil. 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. 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). The dough should come together and slap against the sides of the bowl. It will be tacky.
• Cover bowl tightly with plastic wrap and let dough rise in a warm spot until doubled, about 1 1/2 hours.
• Shape the focaccia. Coat a 12-x 17-inch sheet pan (sides and bottom) with olive oil. Line with parchment and flip the parchment over so exposed side is oiled. Turn dough onto the baking sheet. Oil or moisten your hands, as dough is sticky, and press out dough until it just about covers the bottom of the pan. 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. 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.
• Preheat oven to 425 degrees after 30 minutes of rising (30 minutes before you wish to bake), preferably with a baking stone in it. Place olive oil, sage and garlic in a small saucepan and heat over medium heat until the ingredients begin to sizzle in the oil. 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. Allow to cool.
• Cut away the tough stems bottoms from the mushrooms and tear large mushrooms into smaller pieces. In a large bowl, toss with salt and pepper and the cooled olive oil mix. With lightly oiled fingertips or with your knuckles, dimple the dough, pressing down hard so you leave indentations. Arrange the mushrooms over the dough. Drizzle on any oil left in the bowl.
• Place pan in oven on baking stone. 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. If you wish, remove the focaccia from the pan and bake directly on the stone during the last 10 minutes. Remove from oven, remove from pan at once and cool on a rack. Arrange the whole sage leaves over the top. If you want a softer focaccia, cover with a towel when you remove it from the oven. Serve warm or at room temperature.