In a large skillet or wok, over medium high heat, add 1 tablespoon sesame oil, sauté the pork and potatoes for 2 to 3 minutes.
Add the onions and zucchini and continue to sauté for 2 to 3 minutes.
Add the black bean paste, remaining tablespoon sesame oil, sugar, and garlic to the pan, stirring to combine. Sauté for 3 to 4 minutes.
Add 1/2 cup of water and bring to a boil. Taste and add more black bean paste if needed. Reduce to a simmer.
Mix cornstarch with remaining 1/2 cup cold water (in a separate small bowl) and pour into the sauce to thicken. If the sauce is too thick, you can add another 1/2 cup of water.
Cook for about 10 to 15 minutes, or until vegetables are tender.
While your sauce is simmering, prepare noodles according to package directions.
To Serve: Place a large helping of noodles in a bowl. Ladle the jajang sauce over the noodles and top with the cucumber garnish. Enjoy!