Drain the oil from the paste and reserve to use with the pork; set the paste aside.
Heat the oil again in a large skillet or wok. Add the diced pork loin, sake, garlic, ginger, and a gentle seasoning of salt and pepper to the pan, and cook until no longer pink.
Add the onion and cabbage, then continue to cook until the vegetables just begin to soften, a minute or two.
Add back the black bean paste and stir until the vegetables and pork are coated.
Add 1 cup water and let simmer for 3-4 minutes.
Add the cornstarch slurry and stir to combine, then cook for 1-2 minutes longer, or until sauce thickens.
Turn the heat to its lowest setting while you make the noodles.
When noodles are ready, divide evenly between three or four bowls, top with the jajang sauce and julienned cucumber (if desired), and enjoy immediately, with red pepper powder on the side if desired.
DONE!
