I threw this together in mere minutes when I got home tonight. Wanna know how?
Set a nonstick skillet to ramp up over medium-high heat. Wash tuna medallions (mine were on sale at Harris Teeter, 5.99/lb), dry them thoroughly. Bone dry, I'm talking. Drizzle with olive oil and sprinkle with salt and pepper. Putting the oil on your fish rather than in the pan means less smoking, it's just smart (thank you, Bill). Slap the dark pink hunks of tuna onto the hot pan, there should be a fierce sizzle.
Chop up two plum tomatoes, most of a Serrano chile (take out seeds and rib if you're a lily-tongued pansy), one scallion, and a stalk of celery. Mix all of this colorful vegetary confetti in a small bowl with about a tablespoon of capers and the juice of half a lemon, salt and pepper. I used my limey green au lait bowl with the chipped edge, for it begged to be pulled from the shelf so to join the party of brightness. (Please, puh-lease don't ever use that ReaLemon stuff that comes in the hollow, plastic orb. It's atrocious, the saddest excuse for lemon juice that ever did exist.)
In the meantime, turn your attentions to the stove top. The tuna should have turned opaque about a third of the way up the meat. No need to time it, just use the eyeballs God put in your head. Isn't that great? Flip and let the other side cook the same amount, whilst admiring the lovely golden sear you've attained. There should be a nice little pink stripe running laterally through the piece of fish, it's what will give the flesh the luxurious, buttery texture you want. Don't be afraid of underdone fish, little friend. It will do the opposite of harm you.
Remove from heat, lest they overcook (gasp!). Tumble the tart, fresh salsa of sorts over top or around or below or beside, doesn't matter. Nestle in a little cheek of lemon, too. While I poured my wine, the little piece of citrus had time to bask in the fish's pan-fresh glow, now warm and easy-squeezy, a lovely effect. Spritz, pause, dig, delight.