Puree tomatoes until smooth. Set aside.
Heat beef broth with tomato paste. Set aside.
Make battuto. Chop together the celery, onion and carrots.
Melt butter in large pot until frothy. Add vegetables, sausage and pancetta. Cook on low heat, until veggies are softened, not browned, about 25 minutes.
Add beef, pork, and break up with wooden spoon. Cook 10 minutes. Do not brown.
Add salt and pepper.
Add wine, bring to boil until it is mostly evaporated, about 5 minutes.
Add tomato puree and beef broth mixture and bay leaf.
Cook for 2 1/2 hours on simmering heat until liquid is evaporated.
Add milk and cook for 30 more minutes.
Add pinch of nutmeg. Season with salt and pepper.
Brush rolls with butter. Grill until toasted golden brown.
Spoon bolognese sauce on rolls.