Heat the oil in a very large saucepan; add the onion, garlic and diced aubergine. Cook over a fairly low heat for 5 minutes, until the vegetables are nearly soft. Next, add the diced potatoes, lentils, chopped tomatoes, tomato puree and hot water, along with the dried herbs and plenty of seasoning. Bring to a simmer, and leave to cook fairly gently for around 15 minutes, stirring every few minutes. It doesn't matter if the potatoes and lentils aren't fully cooked in this time, since they will finish off in the oven, but they should be nearly soft.
While the lentil mixture is simmering, make the white sauce. Melt the butter in a different saucepan, and add the flour. Cook over a very low heat, whisking constantly, for one minute to cook the flour. Then add the milk a little at a time, whisking until smooth each time before adding more. When you've added all of the milk, continue whisking for another couple of minutes, still over a very low heat, until the sauce has come together and thickened up a bit. Season well, and add a good pinch of ground nutmeg.
Heat the oven to 190°C (Gas Mark 5 / 375°F).
When the lentils and potatoes are nearly soft, transfer the mixture to a deep baking dish (mine measured 8 x 8 inches), and spread it out into an even layer. Top with the white sauce, and gently spread it out with the back of a spoon to ensure that the whole dish is covered. Top with a few slices of tomato, and bake for around 30 minutes, until the top is golden brown and the mixture is bubbling. Leave to stand for 5 minutes before serving, garnished with fresh coriander if desired.