Mashed Potatoes: Start by boiling the diced potatoes in a pot of salted water until they are tender. Drain the potatoes and return them to the pot.

Mash the potatoes using a potato masher or fork. Add the vegan butter or margarine and plant-based milk. Continue to mash until the potatoes are smooth and creamy. Season with salt and pepper to taste. Set aside.

Filling: In a large skillet, heat a little oil over medium heat. Add the diced onion and cook until it becomes translucent.

Add the minced garlic, diced carrots, and frozen peas to the skillet. Sauté for a few minutes until the vegetables start to soften.

Stir in the cooked lentils, tomato paste, dried thyme, and dried rosemary. Cook for another couple of minutes, allowing the flavors to meld.

Pour in the vegetable broth and bring the mixture to a simmer. Let it cook for about 5-7 minutes until the liquid has reduced and the filling has thickened. Season with salt and pepper to taste.

Assembling the Pie: Preheat your oven to 400°F (200°C).

Transfer the lentil and vegetable filling to a baking dish. Spread it out evenly.

Spoon the mashed potato mixture over the filling, using a fork to create a decorative pattern on top. You can also sprinkle shredded vegan cheese over the mashed potatoes if desired.

Baking: Place the baking dish in the preheated oven and bake for about 15-20 minutes, or until the top is golden brown and the filling is bubbly.

Serving: Once the shepherd's pie is cooked, remove it from the oven.

Garnish with chopped fresh parsley if desired.

Serve the vegetarian shepherd's pie warm, and enjoy this hearty and comforting dish!