In a pot, soak kelp in water for at least an hour (see note)
Bring the pot to a boil. Add apple, onion, garlic, and ginger. Cover, reduce heat to medium low and simmer for about 10 minutes.
Remove kelp. Add coconut aminos and vinegar. Once pot comes to a boil once again cover, reduce heat to medium low an simmer for an additional 10 minutes.
Turn off heat. Using a colander, strain all the remaining juices from apple, onion, garlic, and ginger, pressing out as much liquid as possible.
In a small bowl, combine cornstarch and water. Whisk until smooth. If made in advance, be sure to restir the slurry just before adding it to the sauce as it will separate as it sits.
Turn on heat. Once it comes to a gentle simmer slowly add the slurry to the pot. Whisk continuously until you reach desired thickness, 5-8 minutes. Keep in mind it will continue to thicken once cooled.