Separate leaves from celery stalks and save for another use. Slice celery stalks on the diagonal into pieces ¼-inch wide and 2 inches long. Toss with sea salt and set aside to draw out some water for 30 minutes. After 30 minutes, drain salted water from celery, rinse celery, and squeeze out excess moisture.
Soak sliced onions in ice cold water for about 10 minutes to take off some of the bite.
Make Celery Kimchi
Place drained/rinsed celery, green onions in a large mixing bowl. Drain ice water from onions and add to the mixing bowl is using.
Sprinkle gochugaru, fish sauce, rice vinegar, minced garlic, and grated ginger over the celery and onions. Stir everything to combine. Make sure the celery slices are evenly and well coated; using your hands to gently massage the seasoning into the slices is highly encouraged, but wear gloves!
To eat right away, garnish with toasted sesame seeds. Within the first few hours, the Celery Kimchi will taste like a highly seasoned, spicy salad.
Place the Celery Kimchi into a glass container. Cover with a tightly fitting lid and store in the refrigerator up to 3 days.