Preheat the oven to 375F.
On a lightly floured surface, roll the pastry out to about 10 inches in diameter; transfer to a parchment-lined baking sheet or pie plate if you want to reinforce the sides. In a medium bowl, toss the rhubarb and plums with the sugar, flour, cinnamon and salt (I usually stir the flour into the sugar first to get rid of any lumps), and pile the fruit into the middle of the crust, spreading it out to within a couple inches of the edge. Fold the edge of the pastry over in whatever way it wants to, enclosing the edge of the fruit.
In one of the same bowls, mix together the oats, flour, brown sugar and butter, blending it with a fork or rubbing it between your fingers until well blended. Sprinkle over the fruit.
Bake for about an hour, until the fruit is bubbly and the pastry is golden. Cover it loosely with a piece of foil if the crumble is browning too quickly. Serves 8.