You want to purge the clams: Place the clams in a large mixing bowl. Pour-in enough cold water to submerge the clams. Then sprinkle some kosher salt (1T) into the water. Cover the mixing bowl with aluminum foil or a lid. (They will purge better in darkness). Set aside for at least 1 hour.
In the meanwhile, slice garlic cloves into thin slices. Crumble the dried red chili pepper into small pieces. And dice the spring onion into very thin slices.
After 1 hour, drain-out the water from the clams. Give one final wash and set aside in a strainer.
Take out a large pot or wok. Place it on a medium-high heat. Drop in some butter (2 Tablespoons). When butter starts to sizzle, add in the garlic pieces and chili flakes. Stir it around for 10-15 seconds. Once the garlic pieces start to slightly brown-up on the edges, add-in the clams. Carefully toss the shells in the butter.
Then add in the soju (1 cup). Wait until the soju comes up to boil. Let the soju boil for 1-2 minutes - so it can evaporate the alcohol. Then add-in 1 cup of water. Give it a good stir and let it come back to boil. Then boil for another 1-2 minutes (or until the 90% of the shells open up). At this time, give the broth a taste. If there is a strong alcohol taste, you can boil it for 30 more seconds and/or dilute it with more water (about ¼ cup more).
Move the clams to your presentation bowl. Then pour that delicious broth over the clams. Garnish with the diced spring onion slices and a few shakes of black pepper.