Peel the sweet potato and dice it into small pieces.
Place in a microwave container with a few tablespoons of water and cook on high for four minutes. Drain the liquid and set aside until needed.
Remove meat from the leftover chicken and also set aside until it is needed.
In a heavy-bottomed saucepan, melt the butter and mix in the flour. Remove from the heat, and gradually stir in the milk. When it has combined, return to the heat and stir over a medium to low heat until it thickens. Do not boil.
Remove from the heat and add 1 cup of grated cheese. Season with salt, pepper and mustard to taste.
Add the chicken and sweet potato to the cheese mixture and stir until combined.
Add in the frozen peas
Spread the cooked rice onto the base of a medium-sized baking dish.
Cover with the chicken, sweet potato and pea mixture. Sprinkle with the remaining cup of grated cheese, and scatter breadcrumbs on top.
Bake in a hot oven at around 170C for up to 30 minutes. Serve hot.