Add the red onion and butternut squash to a large rimmed baking sheet. Drizzle with the olive oil and sprinkle with the garlic, salt, pepper and cinnamon. Toss to combine and spread in an even layer. Bake for about 30 minutes, stirring once halfway through, until tender and starting to brown.
As soon as the squash comes out of the oven, add the spinach to the baking sheet and stir to combine. The heat from the baking sheet and vegetables will wilt down the spinach slightly.
For the polenta:
Meanwhile, bring the water, milk and salt to a gentle boil in a medium saucepan. Slowly pour in the polenta in an even stream, whisking constantly the entire time. Turn the heat down to low and partially cover with a lid. Cook for about 15 to 20 minutes, whisking often, until the polenta is thick and creamy. Remove from the heat and whisk in the rosemary and parmesan until combined. Taste and season with salt and pepper as desired.
Divide the polenta into 4 bowls. Top each with the squash mixture and a sprinkle of the pumpkin seeds. Serve immediately.