Heat a large skillet over medium heat.
Add ground beef and onion. Season with salt and pepper
Cook beef until browned.
In a bowl, whisk together garlic, sesame oil, ketchup, hoisin, broth, soy sauce, rice vinegar, and sriracha.
Pour sauce over browned meat and turn heat to medium-low.
Simmer until thickened and sauce is heated through, about 5 minutes.
Toast buns.
Add meat mixture to toasted buns.
Top meat with green onions, sesame seeds, and kimchi.
Serve immediately.