Add the water, mirin, and salt to a container with a lid and stir until the salt is completely dissolved.
Place the salmon in the brine, cover, and refrigerate for at least 12 hours or up to 24 hours.
When you're ready to prepare the salmon, remove it from the brine and use paper towels to pat the fish dry.
You can grill the salmon in a toaster oven set to broil until the fish flakes easily, or you can pan-fry it.
If you are cooking the salted salmon on the stove, heat a frying pan over medium heat and add a small amount of oil. Place the salmon skin-side down and let it fry undisturbed until you can see the salmon turn opaque about 1/3 of the way up the side.
Carefully flip the salmon over and cook on the second side until it's cooked through. You can test it with an instant-read thermometer (it should read 140 F), or you can flake it with a fork (it should flake easily and be opaque all the way through).
While the salmon is cooking, peel and grate the daikon. Drain the pulp in a strainer to remove excess water, and then plate the grated daikon with the salted salmon.