Prepare the lentils and the filling

1. In a medium saucepan, add the lentils and cover with water. Bring to a boil over medium-high heat.
2. Once boiling, reduce the heat to a simmer and cook for about 20 minutes, or until the lentils are tender but not mushy. Drain any excess water and set the lentils aside.
3. Heat the olive oil in a large skillet over medium heat. Add the chopped onions and sauté for 3–4 minutes until they start to soften. Next, add the diced carrots and cook for another 5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
4. At this point, toss in the sliced mushrooms. Sauté them for 5–7 minutes, allowing them to release their moisture and begin to brown. 
5. Add the cooked lentils to the skillet, followed by the tomato paste, soy sauce, thyme, rosemary, and a generous pinch of salt and pepper. Pour in the vegetable broth, stirring everything together to combine the flavors.
6. Let the mixture simmer for 10–15 minutes

Prepare the Mashed Potato Topping

1. Place the peeled and cubed potatoes in a large pot, cover with water, and bring to a boil. Cook the potatoes for 15–20 minutes, or until they are fork-tender. 
2. Drain the potatoes and return them to the pot.
3. Add the vegan butter and plant-based milk to the potatoes. Mash until smooth and creamy. Season with salt and pepper to taste. 

Assemble the Shepherd’s Pie

1. Preheat your oven to 400°F (200°C). Transfer the lentil and mushroom filling into a large baking dish, spreading it out evenly.
2. Gently spoon the mashed potatoes on top of the filling, using a spatula to spread them evenly.
3. Place the assembled pie in the preheated oven and bake for 20–25 minutes, or until the top is golden brown and crispy. 
4. If you prefer an extra-crispy topping, you can switch to the broiler for the last 3–5 minutes, keeping a close eye to prevent burning.
5. Once baked, remove the Shepherd’s Pie from the oven and let it cool for a few minutes before serving.