Preheat the oven to 375 degrees. Line a baking sheet with a silpat mat.
Slice the plums thinly.
Place the plum slices into a bowl and sprinkle the sugar and cornstarch on top.
Gently mix the fruit (I used my fingers) until the sugar and cornstarch are evenly covering them.
Place the plum slices in a spiral inside the pie crust starting one inch from the edge.
Fold the edge of the dough over the fruit.
Brush the dough with the beaten egg then sprinkle the edges with the raw sugar.
Place into the oven and for 40-45 minutes, or until golden brown.
Remove the galette from the oven and let cool for 20-30 minutes before slicing.
Serve plain, with whipped cream, or vanilla ice cream. Enjoy.