Use the tuna sashimi offcuts sold at the supermarket (use 'chuutoro' medium-fatty tuna if you can). The offcuts vary in size, so cut it up into even bite-sized pieces if possible (but you don't have to be precise).
Marinate the tuna in the soy sauce and mirin, and mix well to make sure the flavors are evenly absorbed. You don't need to marinate it for a long time (just a minute is enough).
Once it's evenly mixed, drain off the liquid, add black pepper and mix again. Transfer to serving plates, drizzle with extra virgin olive oil and it's done! I topped with some broccoli sprouts as a garnish.
Story Behind this Recipe
My friend taught me this way of eating tuna, and adapted it to my taste. It's now a family favorite!
Since it goes with Italian or French dishes, you might think that white wine would work better than mirin, but actually nothing beats mirin! Wasabi would work with this too, but don't use garlic or ginger. I think black pepper is the best spice for this.