Preheat your oven to 400°F (200°C).
In a large pot, heat the olive oil over medium heat. Add the chopped onion and cook until translucent, about 5 minutes.
Add the minced garlic and cook for another minute.
Add the mixed vegetables and cook for about 5 minutes until they start to soften.
Stir in the cooked lentils, tomato paste, vegetable broth, thyme, rosemary, salt, and pepper. Let it simmer for about 10 minutes.
While the filling is simmering, boil the potatoes in a large pot of water until tender, about 15 minutes. Drain and return to the pot.
Add the vegan butter and plant-based milk to the potatoes. Mash until smooth. Season with salt and pepper to taste.
Transfer the filling to a baking dish and spread it out evenly. Top with the mashed potatoes, spreading them out to cover the filling completely.
Bake in the preheated oven for about 20 minutes, until the top is golden and the filling is bubbling.
Let it cool for a few minutes before serving. Enjoy!