How to Make It
Put the lentils into a pan, and cover with boiling, salted water. Simmer for 20 mins until tender.
Meanwhile, bring the potatoes to a boil, and then simmer for 15 mins until they are tender.
Heat the oven to 400°F. Heat the oil in a pan; add the onion, and cook for 5 mins to soften. Add the carrots, parsnips, celery, and garlic, and cook for 5 mins.
Add the stock and tomato paste, and simmer for 20 mins. Drain the potatoes, return to pan, season, and add milk and olive oil. Mash until smooth. Drain any excess liquid from the lentils, stir into the vegetable mixture, and then transfer to a large ovenproof dish. Top with the mashed potato. Place in the oven for 20 mins. To serve, garnish with chives.