Sprinkle the aubergine slices with salt and let rest for 60 minutes. Rinse with cold water, pat dry with paper towel. This releases the bitter juices.
In a large pan, heat the olive oil and add the aubergine. Cook the aubergine for 5 to 7 minutes on each side until tender and browned. Remove form the frying pan and place on kitchen paper to soak up any excess oil.
For the tomato sauce:
In the same pan add the onion and cook and stir for 5 minutes. Add the tomatoes with the juice from tin together with the herbs and garlic. Season with salt and pepper. Bring to the boil, reduce heat and simmer for 30 minutes.
For the cheese sauce:
Meanwhile, melt the butter in a saucepan and mix in flour. Stir constantly for 1 minute until the mixture is straw coloured. Add the milk gradually, whisking constantly. Bring to the boil and cook for 2 minutes. Remove from the heat, stir in the cheeses and mustard. Season to taste.
Preheat the oven to 200 C / Gas 6.
Lay half aubergine rounds on the bottom of a baking dish. Then add half of tomato sauce and cover with 3 lasagne sheets. Arrange the remaining aubergine, spread the rest of tomato sauce and place the last layer of lasagne sheets. Pour the cheese sauce over. Cover with tin foil.
Bake for 30 minutes. Remove the foil and bake for another 10 minutes until the top browns.