Step 1: Prepare your ingredients. Chop the aged kimchi into bite-sized pieces. Thinly slice the pork belly. Slice the onion and mince the garlic. Slice the tofu into 1-inch thick pieces. Chop green onions, separating the white and green parts.
Step 2: In a large pot or a deep skillet, heat 1 tbsp cooking oil over medium heat. Add the sliced pork belly and cook until browned and crispy, about 5-7 minutes. Remove the pork belly and set aside, leaving the rendered fat in the pot.
Step 3: Add the sliced onion and the white parts of the green onions to the pot. Sauté for 3-4 minutes until softened and translucent. Add the minced garlic and sauté for another minute until fragrant.
Step 4: Add the chopped aged kimchi to the pot. Sauté with the aromatics for about 5-7 minutes, stirring occasionally, until the kimchi softens and develops a richer color. This step is crucial for developing deep flavor.
Step 5: Stir in the gochujang, gochugaru (if using), soy sauce, and sugar (if using). Cook for 1-2 minutes, stirring to combine everything well. The mixture should be fragrant.
Step 6: Pour in the anchovy broth (or water). Bring the stew to a boil, then reduce the heat to medium-low, cover, and let it simmer for 15-20 minutes, allowing the flavors to meld. The longer it simmers, the deeper the flavor.
Step 7: Gently add the sliced tofu and the previously cooked pork belly to the stew. Continue to simmer for another 5-10 minutes, ensuring the tofu is heated through and has absorbed the stew's flavors.
Step 8: Taste and adjust seasoning if needed. You might want a pinch more soy sauce or sugar depending on the sourness of your kimchi. Stir in the sesame oil just before serving.
Step 9: Ladle the hot Kimchi Jjigae into bowls and garnish with the green parts of the chopped green onions. Serve immediately with steamed white rice and other Korean side dishes.