Make Plum Filling:

Cup of Yum
1. Add the plums, lemon zest and juice, honey, sugar, salt, nutmeg, and rosemary to a heavy bottomed pan (enameled cast iron is best). Cover the pan, and let sit at room temperature for 20 minutes.
2. Place pan over medium heat, and add the butter. When butter melts, reduce heat to medium low. Cook uncovered 15-20 minutes, stirring occasionally.Note: If using a light-weight pan (such as stainless steel), your sauce will cook more quickly. Keep an eye on it.
3. As the plums cook, the sauce will turn bright red, then a dark, rich red. Press down lightly on the plums occasionally to help release their juices. Stir periodically to keep the sauce from burning on the bottom. Turn off the heat when the edges of the sauce begin to brown, and allow to cool for about 10 minutes.
4. Preheat oven to 425°F.

Prepare the Pie Dough:

1. Lay the pie dough on a sheet of parchment paper or a silicone baking mat.
2. Brush the top of the pie dough with the beaten egg.

Assemble the Galette:

1. Scoop out the plum sauce, and place in the center of the pie dough.
2. Begin folding over the dough along the border, smoothing out the overlapping creases with your finger.Tip: Take care to avoid ripping the pie dough when unrolling. If there are any tears in the dough, add a drop of water and smooth the dough out with your fingers. The most important part of folding the galette is to make sure there are no cracks along the bottom edge where filling could leak out.
3. Brush the beaten egg on the folded dough. Sprinkle the galette with sugar.

Bake Galette:

1. Bake 20 - 25 minutes, or until golden.
2. Let galette completely cool for at least 30 minutes before serving.Tip: Avoid moving to a cooling rack until the plum filling has firmed up-- it is very easy to crack the galette (and cause the filling to leak) if the galette is moved at all before the filling sets.

Serve:

1. Serve room temperature or cold. Serve with ice cream or whipped cream (optional).
2. Leftovers:Refrigerate leftover pie and use with 3-4 days.Make Ahead + Freeze:Allow cooked galette to cool completely. Wrap it tightly and freeze. Use within 2 months.To reheat, preheat your oven to 350ºF. Cook for 15-20 minutes until warmed through.