Preheat oven to 400°F (200°C). In large skillet, heat oil over medium heat. Cook grapes, shallots, celery and thyme about 5 min. until softened. Set aside.
In large bowl, combine bread cubes, walnuts, salt and pepper. Mix in grape mixture and vegetable broth, allowing bread to soak up broth. Transfer mixture into 8-in. (20-cm) square baking dish; cover with foil. Bake 30 min. until heated through. Remove foil; bake additional 10 min., or until top is golden brown
Substitute celery with 3/4 cup (175 mL) chopped fennel or leeks.