This is a great make ahead dish, prepare and cook earlier in the day, or the day before. Then allow to cool, cover and place in the fridge until you are ready to reheat. Reheat in a warm oven at 180CFan/160C/360F and warm until hot in the centre. To avoid the potatoes from becoming too brown on the top, leave the dish covered with some kitchen foil. You can also prepare elements of this dish ahead of time, ready to layer up and cook later in the day. Prepare the potatoes, slice them and cover with water to stop them oxidising. Next, cook the garlic and onion in the butter and set aside. When ready to cook, simply assemble and layer up the dish before placing in a hot oven to cook.