Toss cauliflower, squash and onion in a large bowl with oil, garlic powder, 1/2 teaspoon pepper and salt. Spread on a rimmed baking sheet. Roast, stirring once, until tender and browned in spots, 25 to 30 minutes.
Meanwhile, combine broth and water in a small saucepan. Bring to a boil. Slowly whisk in cornmeal, rosemary and the remaining 1/4 teaspoon pepper until smooth. Reduce heat to low, cover and cook, stirring occasionally, until very thick and creamy, 10 to 15 minutes. Stir in cheese; remove the polenta from the heat. Serve the vegetables over the polenta.