Step 1Grind the flour, almonds, sugar and salt in a food processor until the almonds are fine. Add the butter pieces and pulse until the mixture is the texture of coarse crumbs. Sprinkle 2 tablespoons of ice water over the dry ingredients and pulse 4 or 5 times. If the mixture begins to come together, stop now. Otherwise, keep sprinkling the ice water, 1 teaspoon at a time, and pulsing until a dough begins to form.
Step 2Remove the dough from the food processor and quickly and lightly knead on a floured work surface into a solid mass. Press into a disk, wrap in plastic wrap and refrigerate at least 30 minutes.
Step 1Combine the plums and the sugar in a work bowl and toss well to mix. Set aside at room temperature until the dough is chilled.
Step 2Heat the oven to 400 degrees. Remove the dough from the refrigerator and roll it out into a rough circle about 1/4 inch thick. Transfer to a baking sheet.
Step 3Drain any accumulated liquid from the plums. Spoon the plums into the center of the dough. Fold the outside 2 inches of the dough circle toward the center to make a hexagon, leaving the inner 4 or 5 inches of plums uncovered. Scatter the almonds across the plums, then scatter the coarse sugar over the plums and the pastry.
Step 4Bake until the pastry has browned and the plums are cooked through, about 30 minutes.