Start by washing the eggplant and trimming the ends. Then peel them lengthwise but only every other ½ inch. The result should be a "stripes" pattern.
Slice into ½ inch circles and lay on a parchment lined baking sheet in one layer. Add the oil and ensure they are well combined. Bake at 400F for 45 minutes until soft
Meanwhile peel the potatoes and boil for 30-40 minutes until soft when pierced with a fork
Mash using a potato masher and add the butter, milk, and salt and pepper to taste. Mix until soft and creamy
For the meat mixture, saute the diced onions using the oil until soft, about 5 minutes. Add the garlic and the ground beef and cook until the meat is brown, about 5-7 minutes
Add all the other seasoning and the tomato sauce and cook for another few minutes. Taste and adjust the seasoning
Layer the Moussaka with the eggplant first, then the meat mixture, and finally the potato. Add the potato in spoonfuls and gently flatten using a spoon. Create grooves on the top using a fork.
Bake in the oven uncovered at 400F for 25 mins, then broil the top for 5 mins. Allow to cool for 15-20 minutes before cutting and serving.