Mix the beef, breadcrumbs, onion, egg, milk, salt and pepper just until combined and shape the mixture into 18 meatballs.
Heat the oil in a large skillet over medium-high heat and brown the meatballs in two batches, turning to color three sides, about 6 minutes per batch.
Transfer the meatballs to a 5- or 6-quart slow cooker.
Whisk the cranberry sauce, broth, vinegar, brown sugar, Worcestershire and rosemary until mostly smooth and pour it over the meatballs.
Cover and cook on low until the meatballs reach 160°F and the sauce darkens, 4 1/2 to 5 hours.
Turn the meatballs gently through the sauce and let them stand uncovered for 10 minutes before serving.