Method

• Soak the lasagne sheets in hot water for 15 minutes to soften. Drain and refresh under cold water, then drain again.
• Preheat the oven to 200C/180C Fan/Gas 6. You will need a 2.25-litre/4-pint rectangular ovenproof dish.
• Heat the oil in a large non-stick frying pan over a high heat. Add the onion and fry for a few minutes. Add the mushrooms and garlic and fry for 3–4 minutes, stirring, to drive off any excess liquid.
• Sprinkle in the flour and mix well. Add the tomatoes, thyme, vinegar, tomato paste and spinach. Fry over a high heat for a further 3–5 minutes, or until the spinach has wilted and the sauce is bubbling. Season with salt and freshly ground black pepper, then remove from the heat and set aside.
• To make the cheese sauce, melt the butter in a saucepan over a medium heat. Stir in the flour, then whisk in the hot milk. Add the mustard and two-thirds of the cheeses. Season, then bring to the boil, whisking for a few minutes, until thickened.
• To assemble the lasagne, spoon a third of the mushroom and tomato sauce into the base of the dish. Top with a third of the cheese sauce, then place half the lasagne sheets in a single layer on top.
• Repeat the layers until you have three layers of mushroom and tomato sauce, three layers of cheese sauce and two layers of pasta. Sprinkle with the remaining cheeses.
• Bake for about 40 minutes, or until golden brown, bubbling around the edges and the pasta is tender.