Warm sesame oil in a medium-sized pot over medium heat. Once heated, add the beef slices and sauté until they are golden brown and slightly caramelized, approximately 2-3 minutes. Stir occasionally to ensure even cooking.

Incorporate the chopped kimchi into the pot, followed by the gochujang, gochugaru, soy sauce, and minced garlic. Stir well to coat the beef and kimchi evenly with the seasonings. Cook for 5-7 minutes, allowing the kimchi to soften and its flavors to deepen.

Pour in the water or beef stock, stirring gently to combine. Raise the heat to bring the mixture to a boil, then lower it to a simmer. Allow the stew to cook for 20 minutes, letting the beef tenderize and the flavors fully infuse the broth.

Carefully add the tofu cubes and sliced onion to the pot. Let the stew simmer for an additional 5 minutes, ensuring the tofu absorbs the rich broth and the onion softens without losing its texture.

Taste the stew and adjust the seasoning as needed with soy sauce or a pinch of sugar for balance. Just before removing the pot from heat, sprinkle the chopped green onions on top.

Ladle the piping-hot kimchi stew into individual bowls and garnish with fresh cilantro or parsley for a vibrant touch. Pair with a serving of steamed rice for a hearty and satisfying meal.