Combine yeast, flour, salt, tablespoon of sugar, cold butter, egg and lemon zest in food processor. Turn machine on, let it run for a second until mixture is blended, and then let machine run while you add 1/4 cup water through feed tube, a little at a time, until mixture forms a ball and is slightly sticky to the touch. If it is dry, add another tablespoon or 2 of water, and process for another 10 seconds. (In the unlikely event that mixture is too sticky, add flour, a tablespoon at a time.)

Turn dough onto very lightly floured work surface, and knead by hand for a few seconds to form a smooth, round ball. Place in a bowl, covered with plastic wrap or damp cloth, and let rise in warm, draft-free area until dough just about doubles in size, or at least 1 hour. (You can also let dough rise more slowly, in refrigerator, for as long as 6 or 8 hours.)

Preheat oven to at least 500°F (600 is better, if your oven goes that high). Knead dough lightly, and place it on a very lightly floured surface; sprinkle it with a little more flour, and cover with plastic wrap or towel. Let it rest while oven heats.

Pat or roll out dough as thinly as possible to a diameter of 12 inches, using a little more flour if necessary. The process will be easier if you allow dough to rest occasionally between rollings. If you have a pizza stone in your oven, place dough on a floured peel, or long-handled board; if you do not, lay dough on lightly buttered baking sheet.

Spread dough with softened butter, and sprinkle it with remaining sugar. Bake until crust is nicely crisp and sugar lightly caramelizes, about 10 minutes; if galette is browning unevenly, rotate it back to front about halfway through cooking time. Serve hot or at room temperature as a snack, or at room temperature with creme fraiche and cut-up ripe fruit.