Heat half the oil in a large frying pan over medium heat. Add the onion and cook, stirring, for 5 mins or until the onion softens and is golden brown. Add the garlic and rice and cook, stirring, for 1 min or until the grains appear glassy. Transfer the rice mixture to a slow cooker.
Heat the remaining oil in the pan over medium-high heat. Add the pumpkin and cook, without stirring, for 2 mins. Cook, stirring, for a further 2 mins or until lightly browned. Transfer the pumpkin to the slow cooker.
Add thyme and stock to the slow cooker and stir to combine. Cover and cook for 2¾ hours on low or until rice is tender and risotto is creamy, stirring 3-4 times in the last 45 mins of cooking.
Remove thyme sprigs and discard. Add the spinach and parmesan and stir to combine. Divide the risotto among serving bowls. Season and top with extra parmesan to serve.