Make the pickled onions: Bring vinegar, water, sugar, and salt to a simmer, stirring to dissolve. Pour over sliced onions in a jar. Let sit 30 minutes, then refrigerate.
Sear the short ribs in olive oil over medium-high heat for 2 to 3 minutes per side. Transfer to the slow cooker.
In the same pan, cook onion until soft, about 3 to 4 minutes. Add garlic and cook 1 minute. Add spices, pour in broth, scrape up browned bits. Pour over short ribs.
Cook on low for 7 to 8 hours or on high for 4 to 5 hours, until the meat is very tender and pulling away from the bone.
Remove the short ribs, discard the bones and excess fat, and shred the meat with two forks. Return to the cooking juices.
Set everything out and let everyone build their own bowl with rice, beef, pickled onions, and toppings.