Note: if you don't have ready-cooked Brussels sprouts, you can cook them quickly in the microwave. Just trim base, remove outer layer, cut in half and put all pieces in a microwavable container with a little water. They should cook in around 2 minutes on high. You can use ready mashed, boiled, baked or roasted potatoes for this. Just trim off any overly crisp parts and skin then mash up.