If tuna sashimi is in good quality, it is best to eat it simply with wasabi and soy sauce. If you like to have tuna sashimi in different ways or the quality of tuna sashimi is not really good (often in my case), you could make something different. I already posted a few variations on this theme.
This particular night, I made tuna tartar, which is a fairly common Western adaptation of tuna sashimi. Again, this is not a recipe per se, you could make many variations on this theme to your liking. I just chop tuna sashimi in small cubes, mix in finely chopped chives, soy say sauce with wasabi dissolved, a dash of good quality olive oil to your taste. If you like, you could use, chopped perilla and/or scallion, tabasco, sesame oil etc for variations.
To make the taste a bit more interesting, I made a sweet vinegar miso sauce with Japanese mustard “karashi sumiso” からし酢みそ in addition. I used “saikyo” miso 西京味噌, which is sweet to begin with. I used 1 tbs of saikyo miso, 1/3 tsp of prepared hot Japanese mustard (in a tube), 1/2 tsp sugar, and added rice vinegar until a nice smooth saucy consistency is reached. I did this in a Japanese mortar “suribachi” すり鉢 and pestle すりこぎ but you could use a small bowl attachment using a food processor or you could just buy a premade “sumiso” sauce. Taste and adjust the degree of sweetness by adding more sugar.
For assembly, I used a ring mold; tuna tartar on the bottom, thinly sliced mini-cucumber next and thinly sliced black sweet seedless grapes (I just happened to have them) on the top with a garnish of salmon roe “ikura” いくら and chives. I drizzled sumiso sauce on the top and around the tartar on the plate. The dish has an interesting combination of texture, and taste. Mixing sumiso sauce will add one more dimension of nutty sweet and sourness. You could use thinly sliced onion, jicama or Asian pear instead or in addition. You could also use the yolk of a quail egg in stead of salmon roe. We had this with cold sake.