Classic Italian lasagne

Name: Classic Italian lasagne
Prep time: 40 min
Cook time: 2 h 50 min
Total time: 3 h 30 min
Cuisine: Italian
Ingredients: 250g pack fresh lasagne sheets; olive oil, for greasing; large handful grated parmesan; 110g unsalted butter; 1 celery stick, finely chopped; 1 carrot, finely chopped; 1 small red onion, finely chopped; 25g dried porcini, soaked for 10 mins in hot water, drained and roughly chopped; 1 rosemary sprig, leaves picked and chopped; 1kg trimmed beef or veal flank, finely chopped (ask your butcher to do this for you); 125ml dry white wine; 400g can good-quality chopped tomatoes; 1l full-fat milk; 1 bay leaf; 75g unsalted butter; 125g ‘00’ pasta flour; good grating of nutmeg; 100g parmesan, finely grated; 2 egg yolks