Blend together the tomato, onion, carrot and celery with the garlic, paprika, oregano, some salt and pepper. Transfer to a pan and bring to the boil, then simmer for a couple of minutes while you make the bechamel.
Melt the butter in a pan and whisk in the flour. Season and gradually whisk in the milk on a gentle heat. Add the bay leaf and nutmeg and cook for a few minutes until it thickens.
Line the bottom of a lasagne dish with a layer of potatoes followed by half the pork and half the tomato sauce. Top with some bechamel and repeat with another layer. Finish with a layer of potatoes and bechamel topped with a good grating of Parmesan.
Bake in the oven for half an hour until golden on top.