Preheat your oven to 400°F (200°C).
In a large pot, add lentils and vegetable broth. Bring to a boil, then reduce heat and simmer for 20 minutes or until lentils are tender.
In another pot, cook the diced potatoes in boiling water until tender, about 15 minutes. Drain and mash with plant-based milk and vegan butter. Season with salt and pepper.
In a large skillet, sauté the onions, carrots, celery, and garlic until softened. Add tomato paste, thyme, and rosemary. Cook for another 5 minutes.
Add the cooked lentils and frozen peas to the skillet. Mix well and season with salt and pepper.
Transfer the lentil mixture to a baking dish. Spread the mashed potatoes evenly over the top.
Bake in the preheated oven for 20 minutes or until the top is golden brown.
Let it cool for a few minutes before serving. Enjoy!