Heat the olive oil in a skillet over medium heat. Sauté the chopped onion and minced garlic until fragrant and translucent, about 3-4 minutes.
Add the sautéed onion and garlic mixture to the slow cooker, along with the Arborio rice, pumpkin puree, ground cinnamon, and nutmeg. Stir well to combine all ingredients evenly.
Pour in the warm vegetable broth and stir again. Cover the slow cooker with the lid.
Set the slow cooker to low and cook for about 3 hours, stirring once or twice during cooking to prevent sticking and to promote even moisture absorption.
Check the risotto; it should be creamy and the rice tender but not mushy. If needed, add a splash more broth and cook a little longer.
Stir in the grated Parmesan cheese, if using, and season with salt and pepper to taste. Mix until the cheese melts and the risotto is smooth.
Serve the pumpkin risotto hot, garnished with extra Parmesan or fresh herbs if desired. Enjoy the creamy, aromatic dish that’s perfect for a cozy evening.