Chop the rind: Cut the rind in 2-inch wide pieces. If you sliced your whole watermelon into half-moon slices and ate out all the red fruit, that’s basically what you’d have left. Trim off the thin layer of dark green skin. You’ll have to compost it. It’s really too tough to eat. And cut off any dark red pulpy fruit (you can snack on any sweet watermelon trim). It’s fine to leave a thin layer of pink. Cut down the strips of rind into ¼-inch thick, 2-inch wide slices.
Salt the rind: Toss the sliced watermelon and salt in a big bowl. Massage it pretty aggressively with your hands until water starts to seep from the pieces. Walk away and do something else for an hour or two. When you get back, the pieces of rind will be swimming in water. Drain it off and fill the bowl with cold water. Whoosh it all around with your hands and pour off the water. Do this three more times. Then scrape the drained pieces of rind on to a clean kitchen towel and squeeze gently to get rid of most of the remaining water. Put the towel-wrapped rind in the sink for a minute.
Make the kimchi: Rinse out the bowl and mix the garlic, ginger, sugar, Gochujang and Gochugaru in the bowl. Add the squeezed-out watermelon rind, the radishes and greens, and scallions then mix everything together. Let it sit for about 20 minutes until it releases some juice. Then pack it in a quart jar, pushing everything down so that the juice covers all of the solid pieces. Secure the lid on the jar and let it sit for 2 to 5 days at room temperature to ferment. You can tell it’s done when it’s a little bubbly and the hot spices, salt and pickle flavors taste balanced. Refrigerate for up to 3 months.
In Korea, kimchi is served with almost everything: stirred into soups and stews, eaten along with grilled and roasted meats, as a topping to scallion pancakes or tossed in a stir-fry. You can also use it as sandwich topping, mixed into egg or tuna salad, as a garnish for noodle or rice bowls and stirred into ramen.