Preheat your oven to 400°F (200°C).
In a large pot, cook lentils in vegetable broth until tender, about 20 minutes. Drain and set aside.
In the same pot, heat olive oil over medium heat. Add onion and garlic, cook until softened.
Add mixed vegetables, tomato paste, thyme, rosemary, salt, and pepper. Cook for another 5 minutes.
Stir in cooked lentils. Transfer the mixture to a baking dish.
In another pot, boil potatoes until tender. Drain and mash with milk, butter, salt, and pepper.
Spread mashed potatoes over the lentil mixture in the baking dish.
Bake in preheated oven for 20 minutes, or until the top is golden brown.