Bring a large pot of salted water to a boil. Meanwhile, heat 1 tablespoon olive oil in a large skillet over medium-high heat. Season the tuna with salt, add to the skillet and cook, turning, until browned on all sides, about 2 minutes. Remove to a plate.
Add the remaining 1 tablespoon olive oil to the skillet and reduce the heat to medium. Add the red onion, fennel and celery, season lightly with salt and cook, tossing, until the vegetables begin to soften and brown around the edges, 4 to 5 minutes. Add the capers, oregano and red pepper flakes; cook until sizzling, about 1 minute. Pour in the wine and simmer until reduced by half, about 1 minute. Add the tomatoes and 1/2 cup of the boiling water. Cook, smashing the tomatoes with the back of a spoon, until they begin to break down and the sauce is thickened, 8 to 10 minutes.
Meanwhile, add the pasta to the boiling water and cook as the label directs for al dente. Reserve 1/2 cup cooking water, then drain.
Return the tuna to the skillet. Add the pasta and toss to coat, adding the reserved cooking water as needed to loosen. Divide among bowls and top with the celery leaves and fennel fronds.