Bring a pot of water to a boil. Add potatoes and cook until tender, for about 20 minutes.
Meanwhile, heat oil in a frying pan over medium heat. Add garlic and onions and cook for 1 minute. Add carrots, zucchini, tomatoes and parsley. Add seasonings and stir to combine. Cook for 15 minutes.
Once potatoes are ready, drain them and add in milk. Using potato masher, mush potatoes with milk until smooth and creamy.
Preheat oven to 180°C and grease baking tin. Place zucchini mixture into the bottom of the tin, following with lentils. Top evenly with mashed potato. Bake for 25 minutes or until golden. Serve warm and enjoy.