Oven 400 deg F

In a large frying pan, heat the olive oil over medium heat. Add the chopped onion, carrots, and celery. Cook for 8-10 minutes until softened. Add the garlic and zucchini, cooking for another 3-4 minutes until the vegetables are tender.

Stir in the cooked lentils, chopped tomatoes, and tomato paste. Cook for 2-3 minutes to combine the flavors.

Add the vegetable stock, Worcestershire sauce, and thyme. Season with salt and pepper to taste. Bring the mixture to a simmer, reduce the heat, and cook for 15 minutes, stirring occasionally, until the filling thickens slightly.

While the filling is simmering, bring a large pot of salted water to a boil. Add the chopped potatoes and cook for 12-15 minutes, or until tender. Drain the potatoes, return them to the pot, and mash with butter and milk until smooth and creamy. Season with salt and pepper.

Transfer the vegetable and lentil filling to a large ovenproof baking dish, spreading it out evenly. Spoon the mashed potatoes over the filling and smooth the top with a spatula. Use a fork to create a pattern on the surface.

Place the assembled pie in the preheated oven and bake for 20-25 minutes, or until the top is golden and crispy.

Remove the pie from the oven and let it cool for a few minutes before serving. Garnish with fresh herbs like thyme or parsley if desired.