With that in mind, what can you do with Tuna steaks?
Cook them as little as you can stand. If you can get sushi-grade tuna, not cooking them at all would be perfectly acceptable. If you do have to cook them, coat each side with black pepper and maybe some salt or seseme seeds, then cook each side with a very small amount of peanut oil on a hot stainless steel pan. This is sort of a poor-man's tuna equivalent of pan-searing a steak. Cook time varies by how much you trust the tuna, but we're talking minutes, and a small number of them. If the fish is good, I want pink in the center!
You can chop up and serve the tuna over vegetables - avocado and cucumber are what I usually go to. Anything that is good in Sushi will do here.
For rice, you can simply prepare a small amount of rice, or make sushi rice (that is, pre-rinse the starch off short-grain rice and coat it in a rice-vinegar/sugar solution at the end). See also risotto below.
For sauce, I usually make some spicy mayonnaise, which is just regular mayonnaise mixed with Sriracha hot sauce and a small amount of cayenne pepper. (You don't have Sriracha? We must never speak to each other again.) Here my inability to measure things is going to cripple the recipe. Basically you want to use a tiny amount of cayenne pepper - it adds a ton of heat and will make the sauce unmanageable very quickly. Add the Sriracha to taste - it brings spice but also a nice tomato flavor.
For sauce you can also reduce rice vinegar and soy sauce (perhaps with some stock) into the pan after cooking the tuna. This doesn't quite work like a traditional fond in that the tuna isn't fatty enough to leave the rich drippings you get off of steak. Still, this can bring a salty flavor to the party that isn't as lethally hot as the spicy mayonnaise.
I usually serve this mess by doing something silly, like piling the tuna on top of the rice with the vegetables all around, e.g. you can play "New york chef" in the comfort of your own kitchen. The taller you stack it, the more points you get. Duct tape is cheating. The sauce can be poured right on or left on the side or safety. (If I've had a few beers before I start cooking and have been liberal with the cayenne pepper I leave the spicy mayo on the side for safety.)
One of the weirder experiments I have done with this recipe was to make sushi risotto - that is, a risotto flavored with sushi-rice flavors (rice vineger, saki, sugar, although I probably ignored the Saki due to it not being on hand). I'm going to have to make the recipe again to remember what went into it, but the trickiest part is to get the two sets of flavors to play nice together. It can be done. I use Alton Brown's recipe as a starting point for Risotto, but in this case I backed off the white wine a lot (perhaps half a cup at most) to avoid clashing with the sushi-related flavors.
(The recipe was quite dreadful mid-preparation, and I thought we'd be ordering take-out, but as the alcohol cooked off the white wine, the whole thing mellowed.)
Anyway, if you can get a "sushi risotto" going, it can serve as a nice base to the tuna, bringing in some contrary flavors and making the rice more interesting.
Pie in the Sky
There are two other ideas for this recipe that I haven't gotten around to cooking yet:
Wasabi cream sauce. I'll have to figure out how to engineer this but the idea is to make a light cream sauce with a bit of wasabi flavor to pour over the rice. (This is a continuation of the whole "sush-like flavors" theme.) Probably this can't be done without using real cream.
Tangy sauce. Hell, I don't even know what this is. Basically at the Elephant Walk over a decade ago I had a raw tuna and vegetable appetizer, with some kind of tangy salty sauce that was just astoundingly good. If I had to speculate, there was fish sauce or oyster sauce in there, but cut with something to make it not lethal.