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.
A note about dashima or kelp - to maximize flavor, pre-soak for at least 30 minutes (longer the better, if you have time) prior to boiling it. You'll notice some white powder on the surface. Don't wash it off as it's the natural flavor enhancer.You also don't want to cook it for too long as it will start to turn the sauce slimy and leaven an unpleasant taste.
Combine 1 tablespoon of cornstarch with 2 tablespoons of cold water. Be sure to whisk well until fully dissolved.
Stir in slowly, whisking continuously until sauce thickens. Keep in mind it will continue to thicken as it cools so be sure to remove from heat before it reaches your desired consistency.
If you're planning to use this sauce as a marinade, add less cornstarch (1-2 teaspoons) and/or cook for shorter amount of time.
If you want to enjoy it as a glaze or sauce then cook for longer.
Again, the sauce will continue to thicken once it cools so DO NOT cook for too long. Otherwise, you'll end up with jello.
To reheat: the sauce will congeal upon refrigeration due to cornstarch. Reheat in a saucepan on a gentle heat and whisk until it thins out a little bit.