In a large pot, cook the lentils in boiling water for about 25 minutes, or until tender. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion, carrots, and celery. Sauté for about 5-7 minutes until softened. Stir in the minced garlic, thyme, and rosemary, and cook for another minute until fragrant.
Add the cooked lentils to the skillet along with the tomato paste and vegetable broth. Bring the mixture to a simmer and let it cook for about 10 minutes. Add in the frozen peas, and season with salt and pepper to taste if needed. Remove from heat.
While the lentils are simmering, boil the quartered potatoes in a separate pot for about 15-20 minutes, or until fork-tender. Drain and return the potatoes to the pot. Mash them with almond milk and vegan butter until creamy and smooth. Adjust salt if necessary.
Preheat your oven to 400°F (200°C). Spread the lentil mixture evenly into a large baking dish. Top with the mashed potatoes, smoothing them out with a spatula.
Bake in the preheated oven for about 20-25 minutes, or until the mashed potatoes are slightly golden and crispy on top.
Let cool for a few minutes before serving. Spoon out portions, and enjoy your hearty vegan shepherd's pie!
Notes
Feel free to add other vegetables or spices to customize the flavor.