Preheat the oven to 350 degrees.
Dissolve the yeast in the water and let rest in a warm place for 10 minutes.
Put the soft butter and ½ cup of the sugar into the bowl of an electric mixer. Using the paddle attachment, mix on medium speed until smooth. Add the yeast and the egg, mixing well. On low speed, stir in the dry ingredients and the lemon zest and mix just until combined. The dough will be very sticky.
Sprinkle 2 tablespoons of sugar in the middle of a sheet pan. Place the dough on top of the sugar and sprinkle 1 tablespoon of sugar over the dough. Gently roll the dough out into a 10-inch circle. If the dough sticks to the rolling pin, sprinkle the remaining tablespoon of sugar on top of the dough and continue to roll it out. Cut the 1 ounce of cold butter into small pea-size pieces and place them on top of the galette.
Bake the galette for about 15 to 20 minutes, until golden brown.
Slice the galette into wedges and serve it warm.