To a bowl, add the brown sugar, salt, garlic powder, onion powder, ground fennel, allspice and some black pepper and mix to combine.
Rub the short ribs with the mixture and place in an airtight bag or container and refrigerate for 6 hours or up to overnight.
Preheat the oven to 450 degrees F.
Add the onions and garlic to a medium roasting pan or braiser and drizzle with the olive oil. Toss to coat in the oil.
Place the short ribs on top of the onions and garlic.
Bake until the top of the meat begins to brown, about 20 minutes.
Reduce the heat to 325 degrees. Add the beef broth (it should reach halfway up the ribs; add more broth or water if necessary). Cover the pan with foil or a lid and bake until the meat is tender and easy comes off the bone with a fork, an additional 2 1/2 hours.
Remove the beef short ribs and wrap them with tin foil to rest.
Strain the remaining liquid into a saucepan. Bring the remaining liquid to a boil. In a small bowl, add 2 forkfuls of cornstarch with 3 tbsp. cold water. Add this mixture to the liquid, whisk until thickened. Taste and season with salt and pepper. Ladle the sauce on top of the short ribs. Serve with mashed potato