Place the rhubarb in a large mixing bowl with 2 1/2 cups of the sugar and stand until the sugar dissolves, about 1 hour. Preheat the oven to 425 F. Roll out the puff pastry and line a 10-inch tart pan with it. Bake blind for 20 minutes. Cool on a rack. Pour the rhubarb mixture into a large saucepan and bring to a boil. Reduce heat and simmer until thickened. Cool. Beat the egg whites to the soft-peak stage, adding the sugar a little at a time until stiff. Fill the pie shell with the cooled rhubarb mixture. Spread the meringue mixture over the rhubarb and sprinkle with the almonds. Bake for 10 minutes, or until the meringue is golden brown. Cool completely before serving.
