Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter and oil in a large frying pan, add the onion and fry, stirring, for 5 minutes or until softened.
Stir in the garlic, beetroot and rice and cook for 1 minute, then mix in the wine and stock. Add the thyme and a little salt and pepper and bring to the boil, stirring.
Pour the mixture into the slow cooker pot. Cover with the lid and cook on low for 1¾–2 hours or until the rice is tender and almost all the stock has been absorbed.
Spoon into bowls and top with spoonfuls of mascarpone mixed with thyme, a little extra pepper and a generous sprinkling of Parmesan shavings.