Sear the Beef
Preheat oven to 325°F (165°C). Pat beef dry and season with salt and pepper. Heat oil in a Dutch oven over medium-high heat and sear beef on all sides until deeply browned. Remove and set aside.
Build the Base
Add sliced onion to the same pot and cook until softened. Stir in garlic and cook until fragrant.
Deglaze
Pour in beef broth and balsamic vinegar, scraping up any browned bits. Stir in cranberries and brown sugar.
Braise
Return beef to the pot and add herbs. Cover and transfer to the oven. Braise for about 3 hours, until beef is fork-tender.
Reduce the Glaze
Remove beef and simmer the sauce uncovered on the stovetop until thickened into a glossy glaze.
Serve
Slice or shred beef and spoon cranberry balsamic glaze generously over the top.