Trim leeks to the width of the lasagne dish and cut lengthways to the middle.
Slice the inner leek hearts and separate the large outer leaves to use as pasta sheets.
Chop half the onion, wedge the other half, slice celery, and dice carrots.
Fry beef, sliced inner leeks, chopped onion, celery, carrots, and garlic for 10 minutes.
Add mushrooms and cook for 3 minutes until the pan is dry.
Stir in flour, then slowly add red wine, stock, chopped tomatoes, tomato purée, oregano, and one bay leaf.
Simmer the meat mixture for 30 minutes until thick and season with pepper.
Infuse milk with onion wedges and a bay leaf over low heat for 10 minutes.
Mix cornflour with 45ml of milk to create a smooth paste.
Boil the large leek leaves in water for 5 minutes until tender, then drain and cool.
Preheat the oven to 200°C.
Strain infused milk, return to heat, and whisk in cornflour mixture until thick.
Season white sauce with nutmeg and pepper.
Layer a third of the mince and leeks three times in a 2.5 litre dish.
Top with white sauce, sliced tomatoes, and mixed cheeses.
Bake for 30 minutes until golden.