Boil potatoes until tender; mash with oil, milk, salt, and pepper.
Sauté mushrooms, carrots, and onions until browned.
Stir in tomato paste, lentils, broth, and spices. Simmer until thick.
Spread filling in a dish, top with mash, and make ridges with a fork.
Bake at 400°F (200°C) for 20 minutes until golden. Garnish with chives.