Soak beef short ribs in a large bowl of cold water to remove residual blood, about 30 minutes.
Place short ribs, onion, ginger, and 2 cloves garlic in a large pot and cover with water. Bring to a boil, skimming any foam that floats to the top, 20 to 30 minutes. Measure out 2 cups of broth and reserve. Drain short ribs and rinse under cold water; discard onion, ginger, and garlic. Cool until easily handled.
Make slits in each short rib to remove or cut away excess fat.
Return short ribs to the pot. Add reserved broth, soy sauce, brown sugar, red pepper flakes, 4 minced garlic cloves, rice vinegar, and sesame oil. Simmer over low heat until flavors combine, about 45 minutes. Add potatoes, carrots, mushrooms, chestnuts, and dates. Continue simmering until potatoes are tender and sauce thickens, about 10 minutes.
Stir corn syrup into the sauce. Simmer until sauce has a slightly syrupy consistency, about 5 minutes more. Garnish with green onion slices and serve.