Preheat your oven to 400°F (200°C).
In a large pot, bring lentils and vegetable broth to a boil. Reduce heat and simmer for 20 minutes, or until lentils are tender. Drain and set aside.
In the same pot, heat olive oil over medium heat. Add onion and garlic, and sauté until softened.
Add carrots and cook for another 5 minutes. Stir in cooked lentils, peas, tomato paste, thyme, rosemary, salt, and pepper. Cook for another 5 minutes.
Meanwhile, in another pot, boil potatoes until tender. Drain and mash with plant-based milk, vegan butter, salt, and pepper.
Transfer lentil mixture to a baking dish. Spread mashed potatoes evenly on top.
Bake for 20 minutes, or until the top is golden brown. Let cool for a few minutes before serving.