Pat meat dry with paper towels. Sprinkle it with salt and pepper to taste.
In a large, heavy skillet over medium-high heat, melt the butter with the oil. Brown the meat on one side. Turn it over and scatter the vegetables around the meat. Cook until the vegetables are golden.
Transfer the meat to the slow cooker. Add the milk to the skillet with the vegetables and bring it to a simmer. Pour the contents of the skillet over the meat. Cover and cook on low for 3 to 4 hours, or until an instant-read thermometer inserted in the center of the meat measures 160 degrees.
If the sauce is too thin pour it into a saucepan and reduce it over medium heat. Remove the twine. Slice the meat and arrange the slices, overlapping, slightly, on a platter. Spoon the sauce down the center and serve hot.