Preheat oven to 350° F and grease a 6-cup shallow baking dish (or a 9 inch round cake pan).
To make the filling, in a large bowl stir the rhubarb and pears with the sugar, jam, orange juice, corn starch and spices until coated. Transfer the mixture into the prepared baking dish.
To make the topping, combine the flour, oats, brown sugar, cinnamon and salt in a medium bowl. Cut the butter in with a pastry blender, two knives, or by using your hands until uniformly blended and large crumbs form. Sprinkle the crumb topping over the rhubarb mixture.
Bake 40-50 minutes until the crumb topping is golden brown and crisp, the juices bubble and the fruit is tender.