Skinny beef lasagne

Name: Skinny beef lasagne
Ingredients: 2 large leeks, each about 300g/10½oz; 1 medium onion; 2 celery sticks, trimmed; 2 carrots, peeled; 500g/1lb 2oz lean minced beef; 2 garlic cloves, peeled and crushed; 150g/5½oz chestnut mushrooms, wiped and chopped; 2 tbsp plain flour; 150ml/¼ pint red wine; 200ml/7fl oz beef stock, made with 1 beef stock cube; 400g/14oz can chopped tomatoes; 2 tbsp tomato purée; 1 heaped tsp dried oregano; 2 bay leaves; 500ml/18fl oz semi-skimmed milk; 3 tbsp cornflour; Freshly grated nutmeg, to taste; 50g/2oz extra-mature cheddar, grated; 25g/1oz parmesan, finely grated; 3 medium vine tomatoes, sliced; Freshly ground black pepper