Preheat the oven to 200°C / 400°F / Gas mark 6. Heat about 2 tbsp olive oil in a large frying pan. Add the onions, garlic, peppers and cook gently for about 10 minutes or until just golden brown.
Add the lentils, herbs, tomato puree, tomato ketchup and toss around the pan for a couple of minutes. Add the chopped tomatoes and stock and bring to the boil. Reduce to a simmer and remove any scum from the lentils that comes to the surface. Simmer for about 50 minutes to 1 hour or until the lentils are cooked, remove the bay leaf.
Meanwhile, preheat the grill. Brush the aubergine slices with the remaining olive oil and season. Cook the aubergine in batches until tender, soft and golden brown about 10 minutes on each side.