Put the the live blue crabs to sleep by placing them into the freezer for 2 to 4 hours. This step will make the cleaning and preparation process easier.
In the meantime, prepare the marinade. To a medium-sized pot, combine soy sauce, mirin, oligodang syrup, apple, onion, scallion, garlic cloves, ginger, cheongyang pepper, dried red chili peppers, whole black peppercorns, dried kelp, and water.
Bring to a boil over high-heat then reduce the heat to medium. Simmer for 30 minutes then remove from the heat to cool completely. Once the marinade has cooled, strain then set aside.
To clean and process the crab, flip the crab so the underside is facing up. Pull the apron back then twist to remove it. Rinse the crab under cold running water then use a soft bristle brush to scrub away any dirt and debris. Due to the nature of the crab's environment, they're quite dirty so make sure to get into the crevices and thoroughly scrub as best as you can (including the legs!).
With the crab upright and facing you, place the non-dominant hand on one side the legs and use the dominant hand to pull back the shell. Don't toss the shell - we'll come back to this.
Using kitchen shears, trim away the gills, mouth, and tips of the legs. The claws have a generous amount of meat so don't let them go to waste! Use a crab cracker to crack the legs which will allow the marinade to seep in. Once cleaned, cut the crab in half down the middle then repeat for the remaining crab.
Take the crab shell and peel/scrape away the black film with your fingers. Be sure not to peel away the crab fat and roe - it's the best part!
Place the crab in a deep tray or bowl then soak in soju for 10 to 15 minutes.
Arrange the crab in a container with an air-tight lid. Top with the garnishes - cheongyang pepper, red chili pepper, onion and lemon.
Pour the chilled marinade over the crab until completely submerged. Place the crab shells upright, making sure they're submerged as well.
Cover with an air-tight lid then transfer to the fridge. Marinate for at least 24 hours or up to 48 hours.
To serve, transfer the crab onto a plate along with the garnishes (the onions are my favorite!).
Enjoy with a bowl of freshly cooked rice.
