Heat the vegetable oil in a medium pot over medium heat.
Add the diced onion and minced garlic to the pot and sauté for 2-3 minutes until the onion becomes translucent.
Add the chopped kimchi and kimchi juice to the pot and cook for another 2 minutes, stirring occasionally.
Add the uncooked rice to the pot and stir well to combine with the kimchi mixture.
Pour in the water, soy sauce, gochujang, sugar, and black pepper. Stir to mix all ingredients evenly.
Bring the mixture to a boil over medium-high heat, then reduce the heat to low and cover the pot with a tight-fitting lid.
Simmer for 15-18 minutes, or until the rice is fully cooked and the liquid has been absorbed.
Remove the pot from heat and let it sit, covered, for 5 minutes to allow the rice to steam.
Fluff the rice with a fork and stir in the sliced green onions.
Serve the kimchi rice hot, garnished with toasted sesame seeds.