Heat the vegetable oil in a large pot or deep skillet over medium-high heat.
Add the ground beef and cook, breaking it apart with a spoon, until browned and cooked through, about 5-7 minutes.
Add the diced onion and minced garlic to the pot and cook until the onion is softened, about 3 minutes.
Stir in the rinsed rice and cook for 1-2 minutes, allowing the rice to toast slightly.
Pour in the beef broth and water, then add the frozen mixed vegetables, soy sauce, hoisin sauce, sesame oil, black pepper, and salt. Stir well to combine.
Bring the mixture to a boil, then reduce the heat to low and cover the pot with a tight-fitting lid.
Simmer for 18-20 minutes, or until the rice is tender and the liquid has been absorbed.
Remove the pot from heat and let it sit, covered, for 5 minutes to steam the rice.
Fluff the rice and beef mixture with a fork.
Serve the beef bowl garnished with sliced green onions and toasted sesame seeds.