How to Make It
In a bowl, mix the capers with the oregano, shallot and 1/2 tablespoon of the olive oil. Season with salt and pepper.
In a skillet, heat the remaining 1 tablespoon of oil until shimmering. Season the steaks with salt and pepper; add to the skillet and cook over high heat, turning once, until browned and medium-rare, about 4 minutes. Transfer the steaks to a platter.
Add the anchovies to the skillet and cook over moderate heat, mashing with a fork until dissolved, about 1 minute. Add the wine and boil until reduced by half, about 4 minutes. Remove the skillet from the heat and swirl in the butter, 1 piece at a time, until blended and smooth. Season the red wine sauce with salt and pepper.
In a medium bowl, drizzle the arugula with olive oil and season with salt. Mound the arugula alongside the tuna. Pour the red wine sauce around the tuna steaks, top them with the caper mixture and serve.