Heat the oven to 180 C / Gas 4 and grease a shallow ovenproof dish.
Make the chicken sauce: Heat the 3 tablespoons of oil in a frying pan, add the onion and fry gently until soft and lightly coloured. Add the bacon and cook for 1 minute.
Sprinkle in the flour and cook for 1 minute, stirring. Remove from the heat and gradually stir in the stock, tomatoes, chicken and tomato puree.
Return to the heat and bring to the boil, stirring constantly, then lower the heat and simmer for 3 minutes. Remove from heat and set aside.
Bring a large pan of salted water to the boil and cook the lasagne sheets with the 1 teaspoon oil for 10 minutes.
Meanwhile, make the white sauce: melt the butter in a saucepan, sprinkle the flour in and stir over a low heat for 2 minutes. Remove from the heat and gradually stir in the milk, return to the heat again and simmer, stirring until thick and smooth. Set aside.
Drain the lasagne and pat dry with kitchen towel.
Spread half the chicken sauce in the bottom of the greased ovenproof dish and sprinkle with salt and pepper. Place 3 sheets of lasagne on top.
Spread the remaining chicken sauce over the lasagne, sprinkle with more salt and pepper and cover with a second layer of lasagne. Pour the white sauce over the lasagne and sprinkle with salt and pepper.
Sprinkle the grated cheese over the top, then bake in the oven for 1 hour until bubbling and golden.
Leave to rest for 15 minutes, then put back into oven for 5 to reheat (this helps to hold the lasagne together).