Preheat your oven to 400°F (200°C).
Heat olive oil in a skillet over medium heat. Add chopped onion and garlic, and sauté until softened.
Add mixed vegetables and cook for a few minutes. Stir in cooked lentils, vegetable broth, tomato paste, thyme, rosemary, salt, and pepper. Simmer for 10 minutes.
Meanwhile, boil potatoes in a pot of salted water until tender. Drain and mash with vegan butter, plant-based milk, salt, and pepper.
Transfer the vegetable and lentil mixture to a baking dish. Spread mashed potatoes evenly on 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!