Pat the mackerel dry with paper towels. Cut into two pieces, score the skin lightly, and sprinkle both sides with salt. Let sit for 10 minutes, then pat dry again.

Peel the daikon thickly, cut it in half lengthwise, and slice into 1cm thick half-moon pieces.

In a frying pan, combine the ingredients marked with ☆ and bring to a boil. Add the mackerel skin-side up, spoon the broth over it, and cook until the skin changes color. Add daikon and soy sauce, cover, and simmer on low heat for 10 minutes.

Add the ingredients marked with ★ and continue simmering, spooning the broth over the fish, until the sauce thickens slightly.