This might be the most popular noodle dish in Korea. I don't know what it is, but there is something about these black noodles that everyone loves, young and old alike. Parents often take their kids out for jajangmyeon on special occasions, such as graduation, last day of school, birthdays, etc. As such, it is a dish that is deeply embedded in most Koreans' childhood memories.
If you have the Korean black bean paste called chunjang, this dish is very easy to make at home. The black bean paste is first fried in oil. This process helps remove the bitter taste of the bean paste. The fried (or roasted) black bean paste is called jjajang. You can buy either one from most Korean markets. A little bit of sugar is important to balance out the bitterness and saltiness of the black bean paste. Pork is the classic option for the meat, but of course you can substitute it with any meat or seafood.
*Note: Use up to 2 tablespoons for thicker sauce.
In a bowl, combine the pork with the rice wine or mirin and grated ginger, if using. Season with salt and pepper, to taste, and set aside.
Add the black bean paste to a small saucepan with 2 tablespoons vegetable oil, the sugar, and oyster sauce, if using. Fry over medium heat for 2-3 minutes, stirring constantly. (If using pre-fried or roasted black bean paste, jjajang, you can skip this process. Just combine the sugar and the oyster sauce, if using, and set aside.)
Bring a large pot of water to a boil over high heat. Meanwhile, heat a large pan with 1 tablespoon vegetable oil over high heat. Add the pork and stir-fry until almost cooked. Add the onion, cabbage, and zucchini and cook until soft, stirring occasionally.
Stir in the black bean paste and mix everything together until all of the meat and vegetables are coated well with the paste.
Pour in the stock or water and bring to a boil. Reduce the heat to medium-high and cook for 4-5 minutes. In a small bowl, dissolve the cornstarch in the water. Stir in and cook briefly until the sauce is thickened. Add more sugar, to taste, if necessary.
Add the noodles to the boiling water. Cook according to the package directions and drain. (Do not overcook. The noodles should have a firm bite to them.) Place a serving size of noodles in each bowl. Spoon the sauce over the noodles and garnish with cucumber matchsticks, if using.