Preheat your oven to 400°F (200°C).
In a large pot, cook lentils according to package instructions. Drain and set aside.
In the same pot, heat some oil and sauté onions, carrots, and garlic until softened.
Add mushrooms and cook until they release their moisture.
Stir in tomato paste, vegetable broth, thyme, rosemary, salt, and pepper. Let it simmer for 10 minutes.
Add cooked lentils to the pot and mix well. Transfer the mixture to a baking dish.
In a mixing bowl, mash the cooked potatoes with plant-based milk, olive oil, salt, and pepper.
Spread the mashed potatoes over the lentil mixture in the baking dish.
Bake for 20 minutes or until the top is golden brown.
Let it cool for a few minutes before serving.