Because it uses frozen puff pastry, making this tart is easier than it looks. Alternating black and red plums to create "stripes" is optional.
Pulse almonds, 1 tablespoon sugar, and the flour in a food processor until ground to a fine meal.
Roll out puff pastry into a 10-by-14-inch rectangle (1/8 inch thick) on a lightly floured work surface. Using a paring knife, score a 1/2-inch border on all sides. Place pastry on a parchment-lined baking sheet, and freeze until firm, about 30 minutes.
Preheat oven to 375 degrees. Whisk together egg yolk and cream. Brush border with egg wash, then sprinkle almond mixture evenly over center of pastry. Evenly alternate rows of black and red plum slices. Freeze tart until firm, about 30 minutes.
Sprinkle remaining tablespoon sugar over top, then bake until edges are golden brown, about 40 minutes. Brush plums with warm jam; let cool. Serve warm or at room temperature.