In a small food storage container, combine the water, mirin, and salt. Lightly whisk the ingredients together until the salt is fully dissolved.
Then, place the salmon fillets in the brine and ensure they’re fully covered. Close the container with a lid and place it in the refrigerator to cure overnight for at least 12 hours.
Remove the salmon fillets from the brine after 12 hours of curing. Use a paper towel to pat them dry.
(Grilling Option) In a frying pan over medium heat, add oil until it’s hot but not smoking. Place the salmon fillets skin-side down and cook for about 2 minutes, or until the salmon appears about 1/3 cooked from the side. Then carefully flip them over and cook the other side for another 2 minutes.
(Baking Option) Preheat your oven to 425°F and place the rack in the center. Bake the salmon on a baking sheet lined with parchment paper for about 10 minutes.