Pulse the chilies in a food processor until finely chopped. You can also chop up all the ingredients by hand.
Finely chop the apple and tomato. Mince the garlic and finely chop the red onion. Set aside for later.
In a large pot, combine chopped chilies, apple, tomato, soybean paste, salt, sugar, and rice vinegar. Bring it to a boil, then lower the heat and simmer for about 5 minutes. (You don’t have to reduce the liquid too much. It will thicken as it cools.)
Add in the garlic, red onion, and MSG. Cook for one more minute, then turn off the heat.
Let the sauce cool completely, then transfer to airtight containers. It keeps in the fridge for up to 3 months, or in the freezer for up to a year.