Preheat the oven to 400.
After the crust has chilled, remove the plastic wrap and place the disk of dough on the parchment paper.
Place another piece of parchment paper over the top and roll the dough out to about a 12 inch circle.
Gently remove the top piece of parchment paper and discard.
Move the parchment paper holding the crust to a large baking sheet.
Place your jam in the center of the circle of dough and use a spoon to spread it out to about two inches from the edge.
Place the plums on top of the jam. You can just pile them up in there or you can arrange them in a fan pattern starting from the center. (Don’t let the plums cover the last two inches of the dough.)
Fold the edges of the dough towards the center, just covering the edges of the fruit and leaving much of the fruit exposed. If you have difficulty getting the dough to come up off the parchment, bend the parchment up and use a knife to assist you getting the dough off.
Sprinkle the remaining 1/4 cup of sugar over the top of the exposed fruit.
Drop the little cubes of butter over the top of the fruit.
Brush the top of the crust with the egg wash, then sprinkle the crust with some sugar. (I like to use the sparkling coarse decorators sugar here, but you can use whatever you have.)
Bake at 400 for 30-35 minutes until it’s golden brown and the fruit is cooked.
Remove from oven and carefully transfer the parchment paper with the galette to a cooling rack and let cool.
Slice and serve!