First make the bechamel sauce. In a saucepan melt butter. Add flour, stir to combine and cook for 3 minutes. Then add the hot milk, and cook whisking occasionally for 3-5 minutes until thickened and lightly coating the back of a spoon. Season to taste with salt, pepper, and nutmeg and set aside. For the meat sauce, in a skillet melt butter. Add onions, carrots, celery and cook, stirring occasionally for 5 minutes. Then add the smoked ham and cook for another 5 minutes. Add ground beef and pork, and white wine, stir to combine and reduce 1 minute. Add plum tomatoes, chicken stock, tomato paste, bay leaf, oregano, salt & pepper to taste. Bring to a boil and simmer 1 hour.
Preheat oven to 350 F. Brush the bottom and sides of a 9x12x13-inch baking dish with butter. Spread a layer of the meat sauce evenly over the bottom of a baking dish. Then add 1 cup of the bechamel sauce-smoothing it over the meat sauce. Add 1/3 of the lasagne sheets. Continue with 3 more layers. Sprinkle top with parmesan cheese. Place in oven and bake for 30 minutes, or until the cheese is bubbling.