Make the marinade for the short ribs by combining the soy sauce, brown sugar, sesame oil, green onions, garlic, and kosher salt in a bowl and whisk until sugar and salt are dissolved. Stir in the sliced apples.

Place the short ribs in a resealable plastic food storage bag and pour the marinade over the meat. Seal the bag, squeezing to remove as much of the air as possible, and turn the bag over several times so that the marinade is evenly distributed. Place the bag in a shallow bowl (to catch any drips) and refrigerate overnight.

While the ribs are marinating, make the Cucumber Kimchi:

Cut the cucumbers in half crosswise, then quarter each half lengthwise.

In a mixing bowl, toss the cucumbers with the salt; transfer to a colander to drain for 1 hour.

In the same bowl that the cumcumbers were tossed in, combine carrots, onoin, Korean chili flakes, sugar, vinegar, and fish sauce and stir to combine. After the cucumbers have drained, stir them into the carrot-chili mixture and set aside at room temperature for at least 12 hours. Refrigerate in a nonreactive jar or other lidded container for up to 1 week before using.

When you are ready to grill the shortribs, remove them from the refrigerator and allow to come to room temperature. Preheat a grill or broiler to high heat.

Remove the ribs from the marinade and discard marinade. Grill the meat on well-oiled grill grates until nicely marked on both sides but still pink insde, 1 to 2 minutes per side (depending on your grill as well as the thickness of the shortribs). Take care not to overcook.

Serve the shortribs immediately, garnished with the Cucumber Kimchi, a sprinkling of sesame seeds, and green onions.