Combine 1 cup all-purpose flour, 1/4 cup cornmeal, 1/2 teaspoon kosher salt and 1/2 teaspoon granulated sugar in the bowl of a food processor. Pulse several times to combine, before adding 1/2 cup cubed cold unsalted butter. Process until the mixture resembles coarse meal, about 10 seconds — at this point it's actually desirable to have a couple pea-sized butter cubes in the mixture. Anything beyond that will make your dough a little hard to work with. With the machine running, pour 1/4 cup ice water through the feed tube in a slow steady stream, until the dough just holds together. DO NOT PROCESS FOR MORE THAN 30 SECONDS, or your dough will be over-kneaded and hard to work with!

Lay out a large piece of plastic wrap on a clean work surface. Turn out the dough onto the plastic wrap, and work quickly to bring the dough together into a small mound, handling it as little as possible. When you've got a mound (it's okay to have a couple stray pieces), wrap the plastic around the mound. Flatten with the back of your palm and refrigerate for at least 1 hour before using. While the dough is chilling, make the marzipan crumble.

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.