In large bowl, mix parsnips, carrots, peppers, fennel, red onion, oil and salt and black pepper. Spread in single layer in roasting pan.
Roast in 375 degree F oven 45 minutes, stirring halfway through cooking.
In large saucepan, combine milk, water, garlic powder, salt and pepper. Bring to a simmer over medium-high heat. Whisk in cornmeal; cook, whisking, until thick and smooth, 3 minutes. If too thick, add a little more hot water. Stir in Parmesan and basil.
Heat oven to 375 degree F. Coat 13 x 9 x 2-inch baking dish with cooking spray. Spread polenta in baking dish. Arrange vegetables over polenta.
Drizzle cream on top. Sprinkle with cheese and thyme. Bake in 375 degree F oven 25 minutes. Let cool slightly before cutting. Makes 8 servings.