You can get a decent frozen tuna steak from trader joes. Just let it defrost in the fridge overnight and...
rice wine vinegar and/or mirin
(or white wine vinegar and lemon juice in my case since I had neither)
Mix this stuff in a zip lock bag, throw in the tuna, and let it marinate on the counter for about 30 minutes. You want it to warm up a little.
Heat a small amount of sesame oil in a skillet on Med-high heat. Once hot, put the tuna in the skillet and sear it for about a minute on each side. Seared tuna is best served seared. Rare. So don't overdo it. Shown here served with jasmine rice and the wasabi mayo sauce.
Soy sauce- just a little bit
This steak was about a pound. Which is easily two servings.