Heat some cooking oil in a wok or large frying pan over medium heat. Add the minced garlic and sauté until lightly golden and fragrant. Add the beef slices and stir-fry until they change color and are almost cooked through.
Crack in the egg and scramble it until just set. Push the beef and egg to one side of the wok, then add the leftover rice and kimchi.
Add the gochujang, soy sauce, sesame oil, mushroom powder, and pepper. Stir-fry everything together, breaking up any clumps of rice, until the rice is evenly coated with the seasonings.
Lastly, add the chopped green onions and stir-fry briefly until everything is heated through and the beef is fully cooked. Serve hot, topped with a sunny-side-up egg, shredded nori seaweed, and a sprinkle of toasted sesame seeds!
Notes
Use day-old rice. Cold, leftover rice works best because it's slightly drier and less sticky. This helps prevent the fried rice from becoming mushy.
Choose well-fermented kimchi. Slightly sour, aged kimchi adds a deeper, tangier flavor to the fried rice. Fresh kimchi won't give you quite the same rich flavor.
Don't overcook the beef. Thinly sliced beef cooks quickly, so stir-fry it just until it changes color. It will continue cooking when you add the rice and seasonings.
Break up the rice clumps. Use your spatula to separate any clumps of rice while stir-frying. This helps the rice cook evenly and absorb the seasonings.
Adjust the spiciness. Add more or less gochujang depending on your preference. Start with a smaller amount if you prefer a milder flavor.
Add green onions last. Stir in the chopped green onions toward the end of cooking to keep their fresh flavor and aroma.