Begin by chopping the pumpkin into small, uniform pieces to ensure even cooking. Measure out all ingredients so you're ready to go.
Set your slow cooker to low heat. Drop the chopped pumpkin into the pot along with the chopped onion and minced garlic. Add the butter on top and pour in the warm vegetable broth.
Cover the slow cooker and let everything cook for about 3 hours, until the pumpkin is tender and the flavors meld together. The mixture should smell fragrant and the pumpkin should be easily mashable.
Remove the lid and stir in the Arborio rice, making sure it’s coated in the flavorful pumpkin mixture. Cover again and cook on low for another hour, until the rice absorbs the liquid and becomes creamy, but still slightly al dente.
Open the lid and gently stir in the grated Parmesan cheese and chopped sage. Season with salt and pepper to taste. The risotto should be rich and velvety, with a bright orange hue from the pumpkin.
Serve hot directly from the slow cooker for easy family-style sharing. Garnish with additional Parmesan or sage if desired, and enjoy the creamy, autumn-flavored comfort in every spoonful.