Stir Fry the Black Bean Paste
In a small nonstick saucepan over medium heat, heat the oil.
Add the Korean black bean paste (chunjang) and stir fry continuously for about 2 minutes until fragrant.
Remove from heat and set aside.
Make the Jajangmyeon Sauce
In a large saucepan or pot, drain excess oil from the fried black bean paste, leaving a light coating of oil in the pan. Add a little more oil if necessary.
Heat over medium-high heat and add the diced pork. Stir fry until lightly browned and no longer pink.
Reduce heat to medium and add the garlic and ginger. Stir fry for 10–20 seconds until fragrant.
Add the onion, zucchini, and cabbage. Cook for several minutes until the cabbage begins to soften.
Add the fried black bean paste back into the pan and stir well until all ingredients are evenly coated.
Pour in the water, oyster sauce, and sugar. Bring to a boil, then reduce to a moderate simmer.
Simmer for 10–15 minutes, adjusting cooking time based on your preferred sauce thickness and saltiness.
Cook the Noodles
While the sauce simmers, bring a large pot of water to a boil. Cook the noodles according to package directions.
Drain the noodles well. If using fresh Korean noodles, rinse briefly with water to remove excess starch, then drain again.
Divide the noodles into serving bowls.
Finish and Serve
In a small bowl, mix together the cornstarch and water to make a slurry. Stir well before using.
Slowly drizzle the slurry into the simmering sauce while stirring continuously. Cook for several seconds until the sauce becomes thick and glossy.
Ladle the hot black bean sauce over the noodles.
Garnish with julienned cucumber and serve immediately with kimchi and danmuji if desired.
Mix the noodles and sauce thoroughly before eating.