Break the 5 ounces marzipan into small pieces and place in the bowl of a food processor. Pulse a few times, before adding 2 tablespoons almond meal through the feed tube, pulsing to blend. Continue pulsing and add 1 tablespoon cold butter and 1 tablespoon granulated sugar. DO NOT PROCESS FOR MORE THAN A FEW SECONDS AT A TIME — stop pulsing when the crumble is in pea-sized clusters.
For the Plum Filling and Putting It All Together:
Dust a clean work surface with a combination of all-purpose flour and cornmeal. Roll out the chilled cornmeal dough to a 12-inch round, about 1/8 inch thick. Transfer the dough to a large, parchment-lined baking sheet and refrigerate until ready to use.
In a small bowl, combine 1/4 cup granulated sugar and zest from 1 medium orange by using your hand to rub the zest into the sugar. This will help release the oils from the zest and make the sugar more fragrant and flavorful. Set aside.
Preheat the oven to 375 (F).
In a large bowl, gently toss together 1 1/2 pounds pitted and sliced plums, granulated sugar and zest (from the 2nd step), 1/4 cup cornstarch and 1/4 teaspoon kosher salt. Measure out half the marzipan crumble and sprinkle it over the fruit mixture, tossing until the marzipan crumbs have evenly dispersed throughout the mixture.
Immediately transfer to the prepared galette dough, arranging the fruit mixture in the center of the rolled out dough but leaving a 2-inch border all the way around the circle. Fold border over the fruit mixture, overlapping when necessary and pressing gently to adhere to the folds.
Crack open 1 egg in a small bowl and gently whisk until the yolk has incorporated into the weights. Brush the crimped border of the galette with the beaten egg. Sprinkle the remaining marzipan crumble over the galette, and transfer to the preheated oven.
Bake for 1 hour, until the crust is golden brown and the juices are bubbling. Check the galette 15 minutes into baking; if the marzipan crumble is browning too quickly, cover the baking sheet loosely with aluminum foil, creating a "tent". Once the galette is finished, transfer the baking sheet to a wire rack and allow the galette to cool on the sheet on the rack. Serve warm.