Instructions

Bring 3 cups water; ½ coarsely copped yellow onion; ½ ounce dried large anchovies, gutted and heads removed; ½ ounce dashima (dried kelp), rinsed and broken into 2-inch pieces; and ¼ ounce dried large-eyed herring to boil in small saucepan. Reduce heat to vigorous simmer and cook for 10 minutes. Strain broth immediately through fine-mesh strainer into liquid measuring cup. Set aside 1 cup to cool completely. Discard solids and remaining broth (or reserve broth for another use).
Peel away outermost layer of leaves from 6 pounds napa cabbage and set aside. Starting halfway down side of each head, cut cabbage in half lengthwise through core. Using your hands, pull split section of core apart to fully separate cabbage into halves. Place each half cut side down on cutting board and cut crosswise into 1½-inch-thick slices. Discard core. Cut larger pieces that contain mostly white rib in half crosswise.
Submerge cut cabbage and reserved outer layers in large bowl or basin of water, separating layers, then transfer to large colander. (This is not to clean cabbage, but rather to coat all pieces with water so salt sticks.) Discard water.
Place large handful of cut cabbage in now-empty bowl to create layer and sprinkle generously with some of salt. Add remaining cabbage and reserved outer leaves to bowl in batches, sprinkling each batch with remaining salt. Toss cabbage to thoroughly distribute salt, then spread it into even layer. Cover and let sit for 1 hour, tossing cabbage halfway through salting (set timer so you don’t forget!).
Meanwhile, transfer reserved 1 cup broth to now-empty saucepan. Whisk 3 tablespoons all-purpose flour into broth until well combined, then bring to boil, whisking constantly. Reduce heat to simmer and cook, whisking constantly, until thickened, about 1 minute. Remove from heat and let cool completely.
Process ½ cup fresh Asian pear juice, 5 tablespoons saeujeot; and remaining yellow onion in blender until smooth, about 20 seconds; set aside.
Transfer cut cabbage and leaves to colander; discard liquid left in bowl and rinse away any remaining salt from bowl. Remove cabbage from colander and, working in batches, submerge cabbage in cold water in bowl, agitate gently to remove excess salt and dirt, and transfer back to colander. Discard water. Repeat rinsing cabbage once more with fresh water. Let drain for 30 minutes. Wash and dry bowl.
Using your gloved hands, gently mix cabbage; cooled flour paste; pear juice mixture; 4 Fresno chiles, stemmed and finely chopped; 3 ounces mustard greens, stemmed and coarsely chopped; 5 green onions, sliced ¾ inch thick; ¾ cup gochugaru; 7 tablespoons minced garlic; 3 tablespoons maesil cheong; 2 tablespoons fish sauce; 1½ teaspoons sugar; and 1½ teaspoons grated fresh ginger in bowl until well combined. Pull out and reserve outer cabbage leaves. Transfer cut cabbage mixture to 4-quart storage container and gently press into even layer. Arrange outer cabbage leaves evenly on top. Cover container and place in 50- to 70-degree location away from direct sunlight. Let cabbage sit until liquid begins to pool around edges of container, up to 24 hours. (This will be temperature-dependent, so we suggest you start checking after 3 hours, especially if your location is on warmer end of range.) Serve. (Refrigerate for up to 2 months; flavor will continue to develop over time.)