1) It's always better to have more mashed potatoes than insufficient mashed potatoes to cover the top. Keep extra mashed potatoes in the refrigerator up to another 2 days for another meal. Alternatively, use a mini ice cream scoop and scoop mini mashed potato balls onto a parchment paper. Bake at 200C / 400F for 15 to 20 minutes until golden-brown and serve as an appetizer.
2) Try to use a ground that has 10-20% fat so that the Meat Filling stays moist.
3) Save the potato water as vegetable stock in other recipes. Let the potato water cool down completely and freeze in small containers.
4) If the beef/pork has rendered a lot of fat, you can drain off the excess fats by using a spoon to scoop them out. Leave enough fats in the Meat Filling to ensure it will stay moist.
5) The pie can be made ahead right up to assembly. Keep the pie covered and keep in the fridge (especially if the weather is warm) until ready to bake. Bake at 200C / 400F for 30 to 40 minutes until the Meat Filling is hot and bubbling. Broil until the top is golden-brown if needed.
6) If you plan to freeze the Shepherd's Pie, allow the Mashed Potatoes and Meat Filling to cool down completely before assembling the pie. Score the top as usual, then freeze the pie, uncovered, for 2 hours. Wrap with 2 layers of aluminum foil and store in the freezer for another day. Thaw the pie overnight in the refrigerator the day before you want to eat it. Bake the pie, still wrapped with aluminum foil, straight from the refrigerator at 190C / 375F for 45 minutes to 1 hour until the Meat Filling is hot and bubbling. Remove the aluminum foil and broil until the top is golden-brown.