Place potatoes in a large pot of water (just covering potatoes) and bring to a boil, cooking until fork tender, approx. 10 minutes. Drain and return to the pot, add the milk, salt and garlic powder if using, and mash with a potato masher. Add more milk if needed and set aside.
Meanwhile, add your cooked or canned lentils and all remaining filling ingredients to the bottom of a 9"x13" baking dish (or similar). Mix well.
Top with the mashed potatoes and spread them out evenly across the top. Then, use a fork to make crisscross pattern across the top. Cover the tray with parchment paper and then aluminum foil.
Bake for 20 minutes, then remove the foil and paper and bake another 10 minutes uncovered. Remove from the oven and let cool for a few minutes before slicing and serving.
Weigh your potatoes for best results. Since potatoes vary greatly in size, weighing them will be the most accurate.If you don't have a kitchen scale and you have to eye ball it, use approx 10 average sized Yukon Gold potatoes.Don't overly mash your potatoes, just until smooth and then spread them over top your lentil and veggie mixture. Overly mashing potatoes can bring them from smooth and fluffy to gummy territory.Once your lentil and veggie mixture is prepared, taste and add more Tamari, salt, pepper or other seasoning, if needed.Leftovers keep refrigerated for 3-4 days, although reheating may cause the potatoes to dry up a little. Add a splash of milk on top of the potatoes before reheating. Or, heat up as a deconstructed shepherd's pie with some broth or milk and mix everything together to re-moisten it.You may freeze an entire uncooked tray and cook from frozen, adding 20 minutes of covered cooking time. Or thaw first and cook as usual.