Put spaghetti into boiling water. Chop the onion into small pieces. Stir fry mince and onion until done. Scoop out excess fat now if necessary. Add slurry and gochujang. Cook until nice, thicc consistency. Add crushed garlic. Add oyster sauce and soy sauce. Add sugar to taste (and more gochujang if needed). When done, mix with a splash of toasted sesame seed oil. Whenever you feel your sauce needs to be thinned a little, just use a bit of the pasta water.
And if you didn't waste any time, your spaghetti should be done right at this moment. Add sauce to pasta, maybe sprinkle a little bit of furikake on top, dig in.