Step 1: Preheat your oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Step 2: In a large pot, bring the vegetable broth and water to a boil over medium-high heat.
Step 3: Slowly pour the cornmeal into the boiling liquid in a steady stream, whisking constantly to prevent any lumps from forming.
Step 4: Reduce the heat to low and simmer, stirring frequently, for about 15-20 minutes (or according to package directions) until the polenta is thick and creamy.
Step 5: Stir in the pumpkin purée, minced garlic, butter, salt, pepper, and nutmeg until well combined.
Step 6: Add the baby spinach in batches, stirring gently until the leaves have wilted into the hot polenta mixture.
Step 7: Fold in half of the Parmesan cheese.
Step 8: Spread the mixture evenly into the prepared baking dish and smooth the top with a spatula.
Step 9: Sprinkle the remaining Parmesan and the mozzarella cheese over the top of the casserole.
Step 10: Bake for 20-25 minutes, or until the cheese is melted, golden brown, and the edges are bubbling.
Step 11: Let the casserole rest for 5-10 minutes before slicing and serving.