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1
Combine the yeast, flour, salt, 1 tablespoon sugar, cold butter, egg, and lemon zest in a food processor.
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2
Turn the machine on, let it run for a second until the mixture is blended, then let the machine run while you add 1/4 cup water through the feed tube, a little at a time, until the mixture forms a ball and is slightly sticky to the touch.
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3
If dry, add another tablespoon or two of water and process for another 10 seconds.
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4
(In the unlikely event that the mixture is too sticky, add flour, a tablespoon at a time.)
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5
Turn the dough onto a very lightly floured work surface and knead by hand for a few seconds to form a smooth, round dough ball.
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6
Place in a bowl, cover with plastic wrap or a damp cloth, and let rise in a warm, draft-free spot until the dough just about doubles in size, or at least 1 hour.
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7
(You can also let the dough rise more slowly, in the refrigerator, for as long as 6 or 8 hours.)
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8
Preheat the oven to at least 500F (600F is better if your oven goes that high) with a pizza stone in place if you have one.
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9
Knead the dough lightly and place it on a very lightly floured surface; sprinkle it with a little more flour and cover with plastic wrap or a towel.
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10
Let it rest while the oven heats.
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11
Pat or roll out the dough as thinly as possible to a diameter of 12 inches, using a little more flour if necessary.
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12
The process will be easier if you allow the dough to rest occasionally between rollings.
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13
If you have a pizza stone in your oven, place the dough on a floured peel or a long-handled board; if you do not, lay the dough on a lightly buttered baking sheet.
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14
Spread the dough with the softened butter and sprinkle it with the remaining sugar.
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15
Bake until the crust is nicely crisp and the sugar lightly caramelizes, about 10 minutes; if the galette is browning unevenly, rotate it back to front about halfway through cooking time.
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16
Serve hot or at room temperature, but within the hour.
