Measure the soy sauce, water, and rice vinegar into a small bowl in that order: 3 tablespoons, 2 tablespoons, 1 tablespoon.
Add the sugar and stir until you can no longer feel grains against the bottom of the bowl, about 20 seconds.
Stir in the gochugaru and minced garlic.
Chop the scallion finely and add it, along with the sesame oil and crushed sesame seeds.
Let the sauce sit for 10 minutes before serving so the garlic and chili flakes soften and stop tasting raw.
Stir once more just before it goes to the table, since the sesame oil separates and floats.