Combine ground beef, ground pork, breadcrumbs, egg, onion, garlic, red pepper flakes (if using), and rosemary in a bowl. Mix gently until just combined.
Form the meat mixture into meatballs using wet hands. A cookie scoop helps to create uniform meatballs.
Brown the meatballs in a preheated oven at 375°F (190°C) for 15 minutes or in a skillet over medium heat with a little olive oil.
In a saucepan, combine cranberry sauce, balsamic vinegar, brown sugar, Dijon mustard, cinnamon, and nutmeg. Bring to a simmer.
Add the browned meatballs to the cranberry-balsamic glaze. Simmer gently for 15 minutes, or until the meatballs are cooked through and the sauce has thickened.
Taste and adjust seasoning as needed. Serve hot.
