Combine the ketchup, vinegar, soy sauce, sugar, sesame seeds, gochujang, black pepper, scallions, garlic and ginger in a saucepan over medium heat. Whisk until well incorporated. When the sauce comes to a simmer, turn the heat to medium-low and keep at a very low simmer until thickened, about 20 minutes.
Remove the sauce from the heat and stir in the sesame oil. Use immediately, or refrigerate in a nonreactive container for up to 2 days.
Cook's Note: This fragrant and spicy sauce is best on flanken-cut beef ribs or pork ribs applied toward the end of their cooking. If you cannot find gochujang, you can use an equal amount of miso paste mixed with chili paste. The results won't be exactly the same, but it will still be tasty.