Fun story behind this recipe:
We were living near the ocean in Japan and had tried regular tataki. I decided to experiment. It was deemed a success.
1/4 cup vinaigrette dressing (olive oil, balsamic vinegar, dijon mustard, salt, pepper, herbs)
1-2 tbsp chopped fresh rosemary (or 2-3 tsp dried)
1/2 lb oily fish: bonito, tuna
Mix vinaigrette with rosemary. Marinate fish for >30 minutes in dressing. Sear for 2 minutes on each side. Let cool. Should still be quite raw inside. Slice thin and serve with extra dressing if desired. Same works for mackerel, but cook all the way through.