Heat olive oil in a large pan over medium heat. When it starts to shimmer, add onion, celery and carrots. Let cook until soft and somewhat translucent, so flavours come together, and become sweet and intense.
Add butter, and ground beef and pork. Increase heat to high.
Cook, stirring frequently so nothing sticks to bottom of pan, until moisture in meat has been released and meat is slightly browned.
Add wine, stir again to pick up any bits on bottom of pan, and cook until most liquid has evaporated.
Add milk and cook until milk has reduced by 3/4.
Stir in passata pasta and salt. Reduce heat to low and let simmer, stirring occasionally, for a few hours to let flavours come together and intensify.