Preheat oven to 375 degrees F. Grease a 3-quart rectangular baking dish; set aside.
In a large roasting pan toss together sweet peppers, carrots, parsnips, fennel, onion, oil, the 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Bake, uncovered, about 45 minutes or until tender, stirring twice.
Meanwhile, for polenta, in a large saucepan combine milk, the hot water, the 1 teaspoon salt, garlic powder, and 1/4 teaspoon black pepper. Bring just to simmering over medium-high heat. Slowly add cornmeal, stirring constantly. Cook and stir until mixture returns to boiling. Reduce heat to low. Cook, uncovered, about 3 minutes or until thick, stirring frequently. Remove from heat. Stir in Parmesan cheese and basil.
Spread polenta evenly in the prepared baking dish. Top with vegetables. Drizzle with cream; sprinkle with Fontina cheese and thyme.
Bake, uncovered, about 25 minutes or until heated through. Let stand for 10 minutes before serving.