Preheat oven to 375°F (190°C).
Cook lentils according to package instructions. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add onion, carrots, and celery. Cook for 5 minutes.
Add mushrooms and garlic. Cook for another 5 minutes.
Stir in thyme, rosemary, tomato paste, and vegetable broth. Simmer for 10 minutes.
Add cooked lentils to the vegetable mixture. Season with salt and pepper.
Meanwhile, boil potatoes until tender. Drain and mash with plant-based milk and vegan butter.
Transfer lentil mixture to a baking dish. Top with mashed potatoes.
Bake for 25-30 minutes until the top is golden brown.