Make the chicken sauce
1.      Heat the oil in a large pan over a gentle heat. Add the onion, garlic, carrot, celery and courgette and fry gently for about 10 minutes, until softened.
2.      Add the chicken and cook until coloured on all sides. Stir in the oregano, tomato purée, chopped tomatoes and stock. Bring to the boil, then turn down to a gentle simmer for around 30 minutes, until the chicken is tender and cooked through.
3.      Add the butter beans with their liquid and simmer for 5 more minutes. Season with salt and black pepper, then turn off the heat.
Tip: if you’re using tinned butter beans rather than jarred, drain and rinse them and add them with the tomatoes and stock in step 2. Tinned beans need the longer cook to soften.
Make the white sauce
4.      Gently melt the butter in a medium pan. Stir in the flour to make a thick paste.
5.      Gradually whisk in the milk, a little at a time, stirring constantly until you have a smooth, thick sauce that still pours, a little like double cream.
6.      Stir in the nutmeg and 75g of the grated cheese. Leave to cool slightly. It will thicken as it cools, so add a splash more milk if it needs loosening.
Build the lasagne
7.      Butter the base of your ovenproof dish and cover it with a layer of lasagne sheets.
8.      Spoon over half the chicken sauce, spreading it right into the corners. Spread a third of the white sauce evenly over the top, then add another layer of lasagne sheets.
9.      Repeat with the rest of the chicken sauce, another third of the white sauce and a final layer of lasagne sheets.
10.  Cover the top with the last of the white sauce. Sprinkle over the remaining cheese and the Parmesan, and finish with a little black pepper.
11.  If you have time, cover and chill for up to 24 hours. This lets the sauces soften the pasta and set into neat layers.
Bake
12.  Preheat the oven to 180°C, fan 160°C, gas 4. Bake uncovered for around 50 minutes, until golden on top and piping hot all the way through.
13.  Leave to stand for 10 to 15 minutes before slicing. It will hold its shape and come out of the dish far more easily.