Prep Ingredients (Time: 0-15 minutes): Cut 200g pork belly into 1/2-inch cubes. Peel and dice 2 cups Korean radish, 1 cup potato, 1 cup zucchini, all into 1/2-inch cubes. Chop 1 onion. Make cucumber matchsticks for garnish.
Fry Black Bean Paste (Time: 15-17 minutes): Heat wok or large pan over medium. Add 2 tbsp vegetable oil. Add 1/2 cup chunjang (black bean paste). Stir-fry 2 minutes until fragrant and slightly darkened. Remove to bowl.
Cook Pork (Time: 17-22 minutes): In same wok, add pork. Stir-fry 5 minutes until browned and fat renders.
Cook Vegetables (Time: 22-27 minutes): Add all diced vegetables. Stir-fry 5 minutes until slightly softened.
Combine and Simmer (Time: 27-37 minutes): Return black bean paste to wok. Add 2 cups water. Bring to boil, reduce heat, simmer 10 minutes.
Thicken Sauce (Time: 37-39 minutes): Mix 2 tbsp potato starch with 1/4 cup water. Stir into sauce until thickened (about 1 minute).
Cook Noodles (Time: 39-43 minutes): Cook 400g fresh jajangmyeon noodles in boiling water 3-4 minutes until chewy. Drain.
Serve (Time: 43-45 minutes): Divide noodles between bowls. Top with sauce and cucumber matchsticks. Mix thoroughly before eating.