Thaw pastry (assuming you cheated with the frozen stuff, but if you made your own, you’re a hero in our eyes), cut into 2 equal pieces, and roll out to 2 rectangles, about 8” x 10” each. Using a sharp knife, score a 1-inch border on the inside of one, but don’t cut all the way through.
Put each piece of pastry on a parchment-lined baking sheet and refrigerate for 30 minutes to an hour.
Heat oven to 400°F.
Mix the egg with the water, and brush over the scored sheet of pastry. Cut 1-inch strips off the bottom and sides of the second sheet, and lay these over the scored sheet to create a raised border all the way around the edges. You’ll have a smaller rectangle of pastry left; cut this into 1-inch strips.
Toss the rhubarb with sugar, vanilla, cornstarch, and white chocolate chips. Spread on the scored sheet of pastry inside the raised border and top with toasted almonds. Use the remaining strips of pastry to weave a latticework on top, and brush pastry all over with the egg-water mixture. Bake 20 minutes, or until golden brown and puffy.
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