1. Combine the sweet potatoes, parsnips, broth and apple juice in a large saucepan set over high heat.2. Bring to a boil. Cover and reduce the heat to medium-low; cook for 10 minutes.
3. Partially uncover and cook for 10 to 15 minutes or until fork tender. Drain off all but 1/2 cup (125 mL) of the broth. Mash the vegetables and reserve.
4. Meanwhile, heat the margarine in a large skillet set over medium-high heat.
5. Add the apples; cook, stirring occasionally, for 3 to 4 minutes. Sprinkle over the thyme, pepper and salt.
6. Continue to cook, stirring often, for 2 to 4 minutes or until the apples are caramelized and softened.
7. Stir into the mashed vegetables; transfer to a greased, 8-cup (2 L) baking dish. (Crumble can be prepared to this point; cooled, covered and reserved for up to 2 days in the refrigerator.)
8. Preheat the oven to 375° F (190° C).
9. Toss the bread crumbs with the walnuts, blue cheese, margarine and brown sugar until crumbly. Sprinkle evenly over the vegetables.
10. Bake for 30 minutes or until the topping is golden.