Plum Crumble

1 1/4 cups all purpose flour 3/4 cup old-fashioned oats 1/3 cup plus 1/2 cup sugar 1/2 teaspoon ground cinnamon 1/2 cup (1 stick) chilled unsalted butter, cut into pieces 1 pound plums, halved, pitted, cut into wedges 1 tablespoon Read on!