Place the diced onion, minced garlic, arborio rice, and cubed butternut squash into the slow cooker.
Pour the vegetable broth over the rice and vegetables, then add the salt and black pepper. Stir once to combine.
Cover and cook on low for 2 to 2 1/2 hours, until the rice is tender and most of the liquid has been absorbed.
Stir in the butter and grated Parmesan cheese until the risotto turns creamy.
Sprinkle the chopped sage over the top, give the dish one final stir, and serve warm.