To make the sweet potato mash: Roast the sweet potatoes in a 190°C/ 170°C fan forced oven for 1 hour or until very soft but not charred. Remove the skin. Transfer the flesh to a bowl, add butter and mash until almost smooth. Add cream if using and season to taste. Cover to keep warm.
To make the mushroom and chicken rissoles: Heat 1 tablespoon oil in a frying pan over medium-high heat. Add shallots and cook, stirring or shaking the pan, until translucent. Add mushrooms, cook, stirring occasionally, until liquid has evaporated. Transfer to a bowl, set aside to cool.
Add chicken mince and thyme to mushroom mixture, season with to taste and mix well. Use wet hands to shape into small rissoles. Heat remaining oil the frying pan over medium heat, cook rissoles in batches for 4-6 minutes each side or until cooked through.
To make the sautéed mushrooms: Heat the oil and butter in a frying pan over high heat. Add mushrooms and cook shaking pan for 2-3 minutes until light golden, add garlic and herbs and cook 1-2 minutes, season to taste.
Spoon sweet potato mash onto serving plates. Top with rissoles and spoon over sautéed mushrooms. Serve.