In a small saucepan add the chicken stock and thyme then bring to a simmer.
In a large saucepan add the butter and olive oil then heat over a gentle moderate heat. Add the chicken (if using), when white all over add the onion then cook until becoming transparent.
To the chicken add the rice and stir, when it has absorbed the butter and oil then add the wine. When the wine is absorbed add the chicken stock a ladle at a time, stirring after each ladle until absorbed.
When about half the chicken stock is gone add the frozen mixed vegetables. Continue adding the chicken stock.
When the rice is soft and the chicken stock gone gently stir in the Parmesan and half the baked pumpkin.