Add the onion, garlic and sliced mushrooms to the slow cooker, and pour over the white wine. Cook on high for 2 1/2 hours, stirring and scraping down the sides after the first hour if you can.
When the vegetables are very soft, add the vegetable stock and rice, and mix well. Cook on high for a further 2 hours, or until the rice is fully cooked.
Add the cream and fresh parsley, and season to taste. Serve immediately.