Boil the potatoes for 12-15 minutes or until fork tender. Drain, add plant milk, olive oil or vegan butter, nutmeg, salt and pepper and using a hand (immersion) blender or a potato masher, puree until smooth and creamy. Set aside.
Heat olive oil in a large deep skillet. Add onions, garlic, carrots and celery. Cook for about 5 minutes, stirring occasionally.
Add mushrooms, tamari or soy sauce, brown sugar and chili. Cook for 8-10 minutes or until all liquid released by mushrooms evaporates.
Add lentils, tomato paste and veggie stock. Stir well, bring to a simmer and cook for 5 minutes or until all excess liquid evaporates.
Preheat oven to 200C (390F). Transfer lentil mushroom filling to a baking dish (8x8 inch or comparable). Top with mashed potatoes, gently spread the puree with a spoon.
Bake for 15 minutes, then broil for 5 minutes or until you have a golden brown crust on top.
Decorate with freshly chopped parsley and enjoy!