Preheat the oven to 200°C/gas mark 6. Heat the oil in a large pan and sauté the vegetables for 10 minutes.
Add the garlic and balsamic vinegar and cook for 1 minute, then add the remaining ingredients and bring to the boil and simmer for 30-40 minutes until the lentils and vegetables are soft and the sauce is thick.
Meanwhile, boil the potatoes in salted water for 20 minutes or until tender. Drain and mash with milk and butter, then stir in most of the Cheddar.
Divide the sauce between four buttered ramekins, top with mash and sprinkle on the remaining cheese.
Bake for 20 minutes in the preheated oven and serve.
Lentils are a good source of protein and iron for vegetarians, and they make a savoury and satisfying base for these pies.