Make the almond crunch : Preheat the oven to 350°. In a bowl, toss the almonds, sugar and water until coated. Spread the almonds on a parchment paper-lined baking sheet. Bake for about 11 minutes, stirring once and rotating the baking sheet, until golden; let cool. Break up any large pieces.
Make the tart : In a large skillet, melt 1 tablespoon of the butter with 3 tablespoons of the brown sugar over moderately high heat for 1 minute. Add the Chambord and bring to a simmer. Add the plums and cook, stirring occasionally, until the edges begin to soften, about 2 minutes. Using a slotted spoon, transfer the plums to a baking sheet to cool. Reserve the juices in the skillet.
In a food processor, grind the almonds until very fine. In a medium bowl, combine the remaining 2 tablespoons each of butter and brown sugar. Stir in the 1/2 cup of ground almonds and the 1 1/2 tablespoons of flour. Add 1 egg and the vanilla and stir until fully incorporated.
On a lightly floured surface, roll out the pastry to a rough 13-by-11-inch rectangle. Cut the pastry in half lengthwise and transfer one piece to a parchment paper–lined baking sheet. In a small bowl, beat the remaining egg. Lightly brush the beaten egg over the pastry on the baking sheet.
Cut out the center of the remaining piece of pastry, leaving a 1-inch-wide rectangular border; reserve the center for another use. Neatly lay the pastry frame on the rectangle on the baking sheet. Lightly brush the border with the beaten egg. Spread the almond mixture over the base of the pastry. Arrange the plums on the almond filling and refrigerate for at least 30 minutes, until chilled.
Preheat the oven to 375°. Bake the tart for 1 hour, until the pastry is golden brown and cooked through. Let stand for 15 minutes, or until cooled slightly. Brush the plums with the Chambord juices. Cut the tart crosswise into slices and garnish with the almond crunch (or sliced almonds). Serve the tart warm or at room temperature.