1. In a large cast-iron or enamel pot, heat butter and olive oil over medium heat.
2. Add onions, and cook until they begin to soften, about 5 minutes. Add celery and carrots, and cook until the vegetables are tender, 8 to 10 minutes. Add ground sirloin and veal, and cook, stirring occasionally, until the meat is no longer pink. Add milk, and cook at a gentle simmer, skimming fat from surface, until the liquid has reduced by half, about 50 minutes.
3. Add wine, and simmer until liquid is reduced by half again, about 40 minutes.
4. Add beef stock, tomato paste, salt, and pepper; simmer gently until sauce thickens, 40 to 45 minutes. If using for lasagna, set aside to cool slightly before assembling. The sauce can be stored in an airtight container in the refrigerator for up to 3 days, or in the freezer for 2 months.