Preheat oven to 325 degrees F.
Pat short ribs dry with paper towel, and season with salt and pepper.
In large dutch oven, heat oil over medium-high heat.
Once oil is hot, add the short ribs, and sear each side for 1-2 minutes at a time, getting a nice brown crust on each side. Work in batches with the short ribs, if your dutch oven does not fit them all.
Remove meat from pot, then deglaze with 1 cup of beef broth, scraping up any bits.
Add sliced onion and minced garlic, stirring and cooking for 1-2 minutes, until the broth is evaporating.
Add beef back to pot, then top with remaining beef broth.
Season well with salt and pepper. I used about 1 teaspoon of kosher salt.
Cover with lid, and place in oven.
Cook for 2 ½ to 3 hours, or until meat is falling off the bone and shreds easily.
Once meat is done, remove it from the pot, leaving the onions and concentrated broth.
Shred the beef using forks, getting rid of extra fat and the bones.
Puree the broth and onion mixture using an immersion blender or a regular blender.
Combine cornstarch with water and whisk well.
With broth back in the pot if using regular blender, add the cornstarch, and cook over medium-low heat, until it has thickened.
Add half of the gravy to the shredded beef, and reserve the other half for drizzling over the sandwiches.
Serve and enjoy!