Heat the olive oil in a large skillet over medium-high heat. Season the short ribs with salt and pepper, then sear them in the skillet for 3-4 minutes on each side until browned. Remove the ribs and set aside.
In the same skillet, add the chopped onion and garlic. Sauté for about 3-4 minutes until the onion is translucent.
In a crockpot, combine the beef broth, red wine, Worcestershire sauce, brown sugar, thyme, and rosemary. Stir well.
Add the seared short ribs and sautéed onion and garlic to the crockpot. Ensure the ribs are submerged in the liquid.
Cover and cook on low for 6-8 hours or on high for 4-5 hours, until the meat is tender and falling off the bone.
If you prefer a thicker sauce, mix the cornstarch and water in a small bowl until smooth. Stir this mixture into the crockpot about 30 minutes before serving, and cook on high until thickened.
Serve the short ribs with the sauce over mashed potatoes or polenta.