In saucepan, bring 3-1/2 cups (875 mL) water to boil; add wild rice and return to boil. Reduce heat, cover and simmer for 40 minutes. Stir in white rice; simmer, covered, for 20 minutes or until tender. Fluff with fork. Stir in broccoli; cook, covered, for about 5 minutes or until broccoli is tender-crisp. Meanwhile, in separate saucepan, melt butter over medium-high heat; cook onion, garlic, mushrooms, thyme, pepper and salt, stirring, for about 7 minutes or until softened and starting to brown. Sprinkle with flour; cook, stirring, for 1 minute. Gradually whisk in vegetable stock and cream; bring to boil, stirring. Reduce heat and simmer, stirring, for 5 minutes or until thickened. Add to rice mixture along with 1/2 cup (125 mL) of the cheese. Transfer to 13- x 9-inch (3 L) baking dish. (Casserole can be prepared to this point, covered with plastic wrap and refrigerated for up to 24 hours.) Mix remaining cheese with bread crumbs; sprinkle over casserole. Cover with foil; bake in 350°F (180°C) oven for 30 minutes or until bubbly. Uncover and broil for 2 to 3 minutes or until golden.