Preheat your oven to 325°F.
Dry the short ribs with paper towels and generously season all sides with Kosher salt and freshly ground black pepper.
Heat half of the oil in a large braiser or Dutch oven over medium heat. Then, add four of your short ribs. Brown them on all sides, even the ends. If you have trouble standing them up, you can lean them against the sides of the pan.
After you've browned the short ribs on all sides, instead of transferring them to a plate, set them in the inside of the lid.
Add the rest of the oil to the pan and let it get hot for a minute or two. Then, add the rest of the short ribs and brown them on all six sides, just like the previous batch. Once properly browned, add them to the others you've placed in the lid. Set them aside while you make the braising liquid.
Cut the red onion into six wedges and add it to the pan. Stir the onion wedges around in the oil, separating the layers and coating them in the oil. Cook for about 5 minutes, stirring occasionally, until they begin to brown and caramelize.
Smash the cloves of garlic to break them open, then add them and the fresh rosemary to the pan. Cook just until fragrant for about a minute.
Add the red wine and balsamic vinegar. Simmer for about 5 minutes, until the liquid is reduced by about half.
Then, add the beef broth, tomato paste, and Worcestershire sauce. Stir to combine everything and let it come to a simmer.
Then return the browned short ribs to the pan, bone side down. Cover and cook in a 325°F oven for 2 hours.
After 2 hours, remove from the oven and add the cranberries. Return to the oven and cook, uncovered, for 30 minutes.
After they are finished cooking, the meat will be falling off the bone, so carefully remove the short ribs from the pan. Then discard the rosemary stalks from the braising liquid and transfer everything to a 4-cup measuring cup, and purée everything together with an immersion blender to make the sauce. If you don't have an immersion blender, you can use a regular blender.
Serve the short ribs on a bed of Mashed Potatoes and top with the puréed sauce. Yield 4 servings.