Heat oil in a large frying pan over medium-high heat. Cook portobello and button mushrooms, stirring often, for 7-8 minutes or until golden and tender. Transfer to a bowl. Set aside until required.
Heat the butter in the same frying pan over medium-high heat until foaming. Add onion, garlic and half the thyme. Cook, stirring, for 5 minutes or until softened. Add the wine and bring to the boil. Add stock and water and bring to the boil.
Place rice in a 4.5 litre (18-cup) slow cooker. Pour over the stock mixture and stir to combine. Cook, covered, on HIGH, for 1 hour - 1 ½ hours or until rice is just tender. Add the chicken and mushrooms to slow cooker and stir to combine. Cook on High for a further 15 minutes. Stir in 40g (½ cup) of the parmesan. Season.
Serve the risotto immediately sprinkled with remaining cheese.
