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1
Stir together 1 2/3 cups lukewarm (105 to 115F) water and yeast in bowl of mixer and let stand until creamy, about 5 minutes.
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2
Add 5 cups flour, 1/4 cup oil, and 2 1/2 teaspoons table salt and beat with paddle attachment at medium speed until a dough forms.
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3
Replace paddle with dough hook and knead dough at high speed until soft, smooth, and sticky, 3 to 4 minutes.
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4
Turn dough out onto a lightly floured surface and knead in 1 to 2 tablespoons more flour.
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5
Knead dough 1 minute (it will still be slightly sticky), then transfer to a lightly oiled bowl and turn dough to coat with oil.
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6
Let rise, covered with plastic wrap, at warm room temperature, until doubled in bulk, 1 to 1 1/2 hours.
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7
Press dough evenly into a generously oiled 15- by 10- by 1-inch baking pan.
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8
Let dough rise, covered completely with a kitchen towel, until doubled in bulk, about 1 hour.
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9
Preheat oven to 425F.
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10
Stir together rosemary and remaining 3 tablespoons oil.
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11
Make shallow indentations all over dough with your fingertips, then brush with rosemary oil, letting it pool in indentations.
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12
Sprinkle sea salt evenly over focaccia and bake in middle of oven until golden, 20 to 25 minutes.
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13
Immediately invert a rack over pan and flip focaccia onto rack, then turn right side up.
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14
Serve warm or at room temperature.
