Lightly grease the slow cooker insert. Heat the olive oil and butter in a skillet over medium heat, then cook the onion until softened. Stir in the garlic and Arborio rice, cooking for another minute until the rice is lightly coated.
Transfer the rice mixture to the slow cooker. Add the pumpkin purée, warm vegetable broth, white wine if using, sage, thyme, nutmeg, salt, and pepper. Stir everything together until well combined.
Cover and cook on LOW for 2½ to 3 hours, stirring once or twice if possible, until the rice is tender and most of the liquid has been absorbed into a creamy consistency.
Stir in the Parmesan cheese and the remaining butter until fully melted and silky. Taste and adjust the seasoning with additional salt and pepper if needed.
Spoon the risotto into serving bowls and finish with chopped parsley, freshly cracked black pepper, and toasted pumpkin seeds for extra texture if desired.
