Heat a large skillet over medium-high heat. Add the ground beef and cook for 5-7 minutes, breaking it up with a wooden spoon until completely browned, crumbly, and no pink remains. Drain off any excess fat, leaving just a thin coating.

Add the sesame oil to the skillet, then add the diced onion, minced garlic, and minced ginger. Cook until the onion is soft and translucent and the mixture is fragrant, about 3-4 minutes.

In a small bowl, whisk together the soy sauce, brown sugar, and rice vinegar until the sugar is completely dissolved.

Pour the sauce over the beef mixture and stir to combine evenly. Allow the sauce to simmer and thicken slightly for 2-3 minutes, creating a glossy coating on the beef.

Remove from heat and stir in the sesame seeds and sliced green onions.

Serve the Korean ground beef mixture hot over a bed of cooked rice (about 3/4 to 1 cup rice per serving). Top with a fried egg or additional sesame seeds if desired.