Preheat oven to 375F.
Once the oven is hot, spread out the chopped walnuts on a baking sheet and bake for 4-5 minutes, or until browned.
Take the dough out of the refrigerator and set it on a piece of floured parchment paper.
Roll out the dough into a rough circle about 14 inches in diameter. Transfer the dough and parchment to a baking sheet and put it all in the refrigerator to chill while you make the filling.
Pit and slice plums into wedges. Halve the grapes.
Add both to a bowl along with the sugar, rum, and cornstarch. Toss to combine.
Arrange the fruit in the center of the dough round, leaving 2-3 inches bare around the edges. Sprinkle the walnuts over.
Fold up the edges of the dough around the filling. Pleat and pat the dough into place.
Whisk together the egg and water. Then brush the egg wash over the crust.
Mix the sugar and cinnamon. Sprinkle the cinnamon sugar over the galette.
Bake the galette for 45-50 minutes, until the crust is browned and the filling is bubbly.
Transfer the galette and baking tray to a cooling rack.
Bring the honey and water to a boil. Then brush the warm honey over the top of the galette, both the filling and the crust.
Let the galette cool to room temperature, then slice into wedges and serve.