Lamb, potatoes and onions all freeze very well. Just transfer any leftovers from the casserole dish into a plastic container and store in the freezer for up to 4 months. To use, defrost in the fridge and then re-heat either in the microwave, or in a saucepan on the stovetop. You can also freeze the dish once you have cooked it (as far as point 2 in the above section "Can I make this dish in advance"). Just wrap the cooled dish in a layer of tinfoil, cover with clingfilm and freeze for up to 3 months. When you are ready to use it, remove from the freezer and allow to defrost overnight in the fridge. Then place the defrosted, uncovered dish in a hot oven until the dish is heated through and the potatoes are crisp and golden.