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1
Boil potato in 2 cups water in medium saucepan until very tender, about 15 minutes.
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2
Strain 1 1/2 cups cooking liquid into large bowl (add more water if needed to measure 1 1/2 cups).
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3
Cool liquid to 105F to 115F.
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4
Mash potato in small bowl until smooth.
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5
Add mashed potato and 1 teaspoon yeast to cooking liquid.
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6
Cover bowl with plastic and let starter stand at room temperature overnight.
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7
Pour 1/4 cup warm water into small bowl.
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8
Sprinkle remaining 1 teaspoon yeast over; stir to blend.
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9
Let stand until yeast dissolves, about 10 minutes.
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10
Stir yeast mixture into starter.
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11
Stir in melted butter, honey and salt.
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12
Mix in whole wheat flour.
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13
Mix in enough all purpose flour, 1/2 cup at a time, to form soft dough.
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14
Turn dough out onto floured work surface.
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15
Knead until smooth and elastic, about 4 minutes.
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16
Form dough into ball.
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17
Butter large bowl.
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18
Place dough in bowl; turn to coat.
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19
Cover with plastic.
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20
Let dough rise in warm draft-free area until doubled in volume, about 1 hour.
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21
Lightly flour 2 large baking sheets.
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22
Punch down dough.
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23
Turn dough out onto floured work surface and knead until smooth, about 3 minutes.
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24
Divide dough in half.
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25
Roll each half between palms and work surface into 14-inch-long loaf.
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26
Transfer to prepared baking sheets.
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27
Cover with dry towel.
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28
Let rise in warm draft-free area until almost doubled in volume, about 30 minutes.
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29
Position 1 rack in center and 1 rack in top third of oven and preheat to 500F.
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30
Using sharp knife, cut 5 diagonal slashes in surface of each loaf.
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31
Brush loaves with egg white.
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32
Bake loaves 10 minutes.
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33
Reduce oven temperature to 400F.
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34
Bake until loaves are brown and sound hollow when tapped on bottom, switching and rotating baking sheets halfway through baking, about 15 minutes longer.
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35
Cool on racks.
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36
(Can be prepared 2 weeks ahead.
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37
Cool completely.
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38
Wrap loaves separately in foil, seal in plastic bag and freeze.
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39
Thaw loaves, then rewarm, wrapped in foil, in 350F oven 15 minutes.)
