Preheat the oven to 400F/200C. Spray a 13 x 9-inch ovenproof dish with non-stick spray.
Arrange the eggplant slices in a single layer on the prepared trays. Drizzle over the oil and season with salt and pepper. Roast for 20–25 minutes turning the slices over halfway through the cooking time. Set aside.
Lay the oven-ready lasagna in a dish and cover with hot water for 10 minutes to soften.
To make the white sauce: whisk the flour and butter for a few minutes over a high heat to form a roux. Pour the warmed milk into the roux and whisk. Bring to the boil, whisking for a few minutes until smooth. Add the mustard and two-thirds of the Parmesan. Season with salt and pepper and stir. Set aside.
To make the tomato sauce, put all the ingredients in a mixing bowl. Season with salt and pepper.
To assemble, spoon a third of the tomato sauce in the base of the prepared dish, top with a third of the béchamel. Arrange a third of the eggplant on top and 3–3½ sheets of lasagne (you may need to break the lasagne into pieces to make it fit in an even layer). Repeat so you have three layers of sauce and two layers of pasta, sprinkle with the remaining Parmesan.
Leave to stand for 1 hour before cooking, or overnight.
Bake for 35–45 minutes until golden-brown and bubbling. Stand for 5 minutes before serving.