Heat olive oil and 1 tablespoon of butter in a large skillet over medium heat. Add diced onion and cook for 5–7 minutes until translucent, then stir in garlic and sage for 60–90 seconds until fragrant.
Transfer the aromatic mixture to the slow cooker insert, scraping all the flavorful oil from the pan.
In the same skillet over medium heat, stir the arborio rice constantly for 2–3 minutes until grains turn slightly translucent at the edges and smell nutty, then add to the slow cooker.
Return the skillet to medium-high heat, pour in the white wine, and scrape up all browned bits; simmer 2–3 minutes to cook off the alcohol, then pour into the slow cooker.
Pour in the vegetable broth and pumpkin puree, then add salt, pepper, and nutmeg; stir thoroughly until completely combined with the rice, ensuring it is evenly submerged.
Cover and cook on HIGH for 2 to 2½ hours, checking at 2 hours; the risotto is done when the rice is al dente and most liquid is absorbed with a creamy, slowly-flowing texture.
Turn off the slow cooker and stir in the remaining 2 tablespoons of butter, Parmesan cheese, and heavy cream (if using); taste and adjust seasoning, then rest uncovered for 5 minutes.
Spoon into shallow bowls and top with a drizzle of olive oil, extra Parmesan, toasted pumpkin seeds, and fresh sage leaves; serve immediately.