Place stock into a saucepan and bring to the boil. Reduce heat, cover and gently simmer.
In a separate pan, heat butter and olive oil over medium-high heat.
Add chicken and mushrooms and cook for 6 minutes until golden brown.
Transfer to a bowl and keep warm.
Add leek and garlic to pan reduce heat and cook gently for about 10 minutes without colouring.
Add rice and cook over medium heat for 3 minutes, stirring frequently.
Stir in wine and cook until nearly all of it has been absorbed.
Add 1/2 cup of warm stock. Reduce heat to simmering and continue stirring until all liquid has been absorbed. Continue to add stock 1/2 cup at a time allowing it to be absorbed before adding more. This will take around 20 minutes.
Return chicken and mushrooms to pan with Tuscan kale (cavolo nero).
Season with salt and pepper and cook for a further 5 minutes.
Remove from heat and add extra butter, parmesan and walnuts. Stir well. Cover with lid and stand for a few minutes.