Pat the short ribs dry and season generously with salt and pepper. Heat avocado oil in a large skillet over medium-high heat. Sear the ribs on all sides until a deep brown crust forms, about 2-3 minutes per side. Transfer the seared ribs to the slow cooker.

In the same skillet, reduce heat to medium. Add the chopped onion and sauté for 3-4 minutes until softened. Add the minced garlic and cook for 1 minute more until fragrant.

Pour in the beef broth, balsamic vinegar, soy sauce, and Worcestershire sauce. Whisk in the brown sugar until dissolved. Bring to a simmer, scraping any browned bits from the bottom of the pan.

Pour the balsamic mixture over the short ribs in the slow cooker. Add the fresh thyme sprigs. Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the meat is fork-tender.

Carefully remove the short ribs from the slow cooker. Strain the cooking liquid into a saucepan and skim off any excess fat. Bring the liquid to a boil and cook for 10-15 minutes, or until it has reduced and thickened into a glaze.

Spoon the rich balsamic glaze over the short ribs and serve hot with your favorite sides.