Heat the vegetable oil in a medium pot over medium heat.
Add the diced onion and cook until softened, about 3 minutes.
Add the minced garlic and cook for 30 seconds until fragrant.
Stir in the chopped kimchi and cook for 2 minutes to release flavors.
Add the uncooked rice to the pot and stir well to combine with the kimchi mixture.
Pour in the water, kimchi juice, soy sauce, sesame oil, sugar, and black pepper. Stir to combine.
Bring the mixture to a boil, then reduce heat to low and cover the pot with a tight-fitting lid.
Simmer for 15 to 18 minutes, or until the rice is cooked and liquid is absorbed.
Remove the pot from heat and let it sit, covered, for 5 minutes.
Fluff the rice with a fork and stir in the sliced green onions.
Serve the kimchi rice garnished with toasted sesame seeds.