In a large, heavy-bottomed pot, warm olive oil over medium heat. Add celery, carrots, onion, and pancetta. Cook until pancetta has browned.

Add beef and pork, breaking the meat apart with a fork. Cook meat and vegetables until dry. Add tomato puree and paste.

Use a piece of kitchen twine to tie rosemary, sage, and basil together. Place bundle into sauce.

Cover and simmer over low heat for 6 hours, stirring occasionally. Remove herb bundle, season with salt and pepper, and stir in butter.