Mix beef, breadcrumbs, onion, egg, milk, salt, and pepper just until combined. Shape into 18 meatballs.
Heat oil in a large skillet over medium-high heat. Brown meatballs in two batches, turning to brown three sides, about 6 minutes each.
Transfer to a 5- or 6-quart slow cooker.
Whisk cranberry sauce, broth, vinegar, brown sugar, Worcestershire, and rosemary until mostly smooth. Pour over meatballs.
Cover; cook on low for 4 1/2 to 5 hours until meatballs reach 160°F and sauce darkens.
Gently turn meatballs through sauce; rest uncovered for 10 minutes before serving.