Heat the oil in a large, nonstick saucepan and sauté onion, carrots, celery and garlic, over medium heat, for 10 mins or until softened
Stir in the tomato paste followed by the red split lentils.
Add the canned tomatoes, balsamic vinegar, Worcestershire sauce, bay leaf and 1 cup of the stock.
Over medium heat simmer for approx. 30 minutes (until the lentils are soft and the sauce has thickened), adding more stock as needed.
Meanwhile, preheat oven to 200c / 400f and place the sweet potato in a pan of boiling water, simmer for approx. 15 mins or until tender.
Drain the potatoes and return them to the pot. Leave for a minute, shaking the pot occasionally to encourage the evaporation of any excess water. Mash thoroughly with a potato masher and season to taste. (Do not season if making for a baby).
Transfer the lentil mixture to a oven proof dish and top with the sweet potato.
Sprinkle with cheese and bake in the oven for approx. 20-30 mins until golden and bubbling.