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.