Prepare the filling: 1.Cook the lentils in water according to the package instructions. Drain and set aside.
2.In a large skillet, heat olive oil over medium heat. Sauté onion and garlic for 5 minutes until soft. Add carrots, peas, tomato paste, and vegetable broth. Simmer for 10 minutes until vegetables are tender. Stir in lentils, thyme, rosemary, salt, and pepper. Set aside.
Make the mashed potatoes: Boil peeled potatoes in salted water for 15-20 minutes until soft. Drain and mash with plant-based milk and vegan butter. Season with salt and pepper.
Assemble the casserole: Preheat the oven to 375°F (190°C). Spread the lentil mixture in the bottom of a casserole dish. Top with mashed potatoes, smoothing them out evenly.
Bake: Bake in the oven for 25-30 minutes, or until the top is golden and slightly crispy.